Salvatore Sciortino
Bibliografia
Refereed
2019
- CSI 2264: Simultaneous optical and X-ray variability in the pre-main sequence stars of NGC 2264. II. Photometric variability, magnetic activity, and rotation in class III objects and stars with transition disksAstronomy & Astrophysics, Volume 628, id.A74, 49 pp.Aug 2019
- The complex phenomena of young stellar objects revealed by their X-ray variability
Abstract
X-ray observations of young stellar objects (YSOs) have shown several complex phenomena at work. In recent years, a few X-ray programs based on long, continuous, and, sporadically, simultaneous coordinated multiwavelength observations have paved the way to our current understanding of the physical processes at work, which very likely regulates the interaction between the star and its circumstellar disk. We will present and discuss some recent results based on a novel analysis of a few selected very large flares observed with the Chandra Orion Ultradeep Pointing, on the systematic analysis of a large collection of flares observed with the Coordinated Synoptic Investigation of NGC 2264 (CSI 2264) as well as on the Class I/II YSO Elias 29, in the rho Oph star forming region, whose data have been recently gathered as part of a joint simultaneous XMM-Newton and NuSTAR large program.
Astronomische Nachrichten, Volume 340, Issue 4, pp. 334-339May 2019 - Wide-area photometric and astrometric (Gaia DR2) study of the young cluster NGC 6530
Abstract
Context. NGC 6530 is a young cluster, with a complex morphology and star-formation history. We present a statistical study of its global properties, using a new, large list of candidate members down to masses of 0.2-0.4 M☉ and Gaia DR2 astrometry. Aims: We consider a larger sky region compared to previous studies, to investigate the entire cluster until its periphery, including any diffuse population all around the main cluster. We study the distribution of extinction and age across the different regions, and obtain constraints on the star-formation history. We also study the dynamics of cluster members. Methods: Cluster membership was determined on the basis of literature X-ray data, Hα emission, near-IR and UV excesses from the VPHAS+ and UKIDSS photometric surveys and published near-IR catalogs, and Gaia DR2 astrometry; moreover, we used a method for photometric selection of M-type pre-main-sequence cluster members, which we recently developed and used for other star-formation regions. The list of candidates includes nearly 3700 stars, of which we estimate approximately 2700 to be genuine NGC 6530 members. Results: Using Gaia parallaxes, the cluster distance is found to be 1325 pc, with errors of 0.5% (statistical) and 8.5% (systematic), in agreement with previous determinations. The cluster morphology and boundaries are established with great confidence, from the agreement between the subsamples of members selected using different criteria. There is no diffuse population of members around the cluster, but there are minor condensations of true members in addition to the two main groups in the cluster core and in the Hourglass nebula. Two such subgroups are spatially associated with the stars 7 Sgr (F2II-III) and HD 164536 (O7.5V). There is a definite pattern of sequential star formation across the cluster, within an age range from less than 0.5 Myr to ̃5 Myr. Extinction is spatially non-uniform, with part of the population still embedded or obscured by thick dust. The precise Gaia proper motion data indicate that the NGC 6530 parent cloud collided with the Galactic plane around 4 Myr ago, and we suggest that event as the trigger of the bulk of star formation in NGC 6530. The internal cluster dynamics is also partially resolved by the Gaia data, indicating expansion of the main cluster population with respect to its center. Full Table 1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz- bin/qcat?J/A+A/623/A25
Astronomy & Astrophysics, Volume 623, id.A25, 21 pp.Mar 2019 - Deep X-ray view of the Class I YSO Elias 29 with XMM-Newton and NuSTAR
Abstract
X-ray emission is a characteristic feature of young stellar objects (YSOs) and the result of the interplay between rotation, magnetism, and accretion. For this reason high energy phenomena are key elements to understand the process of star formation, the evolution of their circumstellar disks, and eventually the formation of planets. We investigated the X-ray characteristics of the Class I YSO Elias 29 with joint XMM-Newton and NuSTAR observations of total duration 300 ks and 450 ks, respectively. These are the first observations of a very young (<1 Myr) stellar object in a band encompassing simultaneously both soft and hard X-rays (0.3 - 10 keV in XMM-Newton and ≈3 - 80 keV in NuSTAR). The quiescent spectrum is well described by one thermal component at ̃4.2 keV absorbed by NH ̃ 5.5 × 1022 cm-2. In addition to the hot Fe complex at 6.7 keV, we observed fluorescent emission from Fe at ̃6.4 keV, confirming the previous findings. The line at 6.4 keV is detected during quiescent and flaring states and its flux is variable. The equivalent width is found varying in the range ≈0.15 - 0.5 keV. These values make unrealistic a simple model with a centrally illuminated disk and suggest a role of the cavity containing Elias 29 and possible reverberation processes that could occur in it. We observed two flares that have durations of 20 ks and 50 ks, respectively, and we observed the first flare with both XMM- Newton and NuSTAR. For this flare, we used its peak temperature and timing as diagnostics to infer a loop size of about 1 - 2 R☉ in length, which is about 20%-30% of the stellar radius. This implies a relatively compact structure. We systematically observed an increase in NH of a factor five during the flares. This behavior has been observed during flares previously detected in Elias 29 with XMM-Newton and ASCA. The phenomenon suggests that the flaring regions could be buried under the accretion streams and at high stellar latitudes because the X-rays from flares pass through gas denser than the gas along the line of sight of the quiescent corona. In a different scenario, a contribution from scattered soft photons to the primary coronal emission could mimic a shallower NH in the quiescent spectrum. In the spectrum of the full NuSTAR exposure, we detect hard X-ray emission in the band ≈20 - 80 keV which is in excess with respect to the thermal emission and that is significant at a level of ≥2σ. We speculate that the hard X-ray emission could be due to a population of energetic electrons accelerated by the magnetic field along the accretion streams. These particles, along with X-ray photons with E > 7.11 keV, could be responsible for pumping up the Fe fluorescence when hitting cold Fe in the circumstellar disk. Based on observations obtained with XMM- Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA.
Astronomy & Astrophysics, Volume 623, id.A67, 14 pp.Mar 2019 - Stellar population of Sco OB2 revealed by Gaia DR2 data
Abstract
Context. The Sco OB2 association is the nearest OB association, extending over approximately 2000 square degrees on the sky. Only its brightest and most massive members are already known (from HIPPARCOS) across its entire size, while studies of its lower mass population refer only to small portions of its extent. Aims: In this work we exploit the capabilities of Gaia DR2 measurements to search for Sco OB2 members across its entire size and down to the lowest stellar masses. Methods: We used both Gaia astrometric (proper motions and parallaxes) and photometric measurements (integrated photometry and colors) to select association members, using minimal assumptions derived mostly from the HIPPARCOS studies. Gaia resolves small details in both the kinematics of individual Sco OB2 subgroups and their distribution with distance from the Sun. We developed methods to explore the 3D kinematics of a stellar population covering large sky areas. Results: We find nearly 11 000 pre-main-sequence (PMS) members of Sco OB2 (with less than 3% field-star contamination), plus ̃3600 main- sequence (MS) candidate members with a larger (10-30%) field-star contamination. A higher confidence subsample of ̃9200 PMS (and ̃1340 MS) members is also selected (<1% contamination for the PMS), however this group is affected by larger (̃15%) incompleteness. We separately classify stars in compact and diffuse populations. Most members belong to one of several kinematically distinct diffuse populations, whose ensemble clearly outlines the shape of the entire association. Upper Sco is the densest region of Sco OB2. It is characterized by a complex spatial and kinematical structure and has no global pattern of motion. Other dense subclusters are found in Lower Centaurus-Crux and in Upper Centaurus-Lupus; the richest example of the latter, which has been recently identified, is coincident with the group near V1062 Sco. Most of the clustered stars appear to be younger than the diffuse PMS population, suggesting star formation in small groups that rapidly disperse and are diluted, reaching space densities lower than field stars while keeping memory of their original kinematics. We also find that the open cluster IC 2602 has a similar dynamics to Sco OB2, and its PMS members are currently evaporating and forming a diffuse (size ̃10°) halo around its double-peaked core. Full Tables A.1 and A.2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz- bin/qcat?J/A+A/623/A112
Astronomy & Astrophysics, Volume 623, id.A112, 25 pp.Mar 2019 - Simultaneous Kepler/K2 and XMM-Netwon observations of superflares in the Pleiades
Abstract
Flares are powerful events ignited by a sudden release of magnetic energy. With the aim of studying flares in the 125-Myr-old stars in the Pleiades observed simultaneously in optical and X-ray light, we obtained new XMM-Newton observations of this cluster during the observations of Kepler K2 Campaign 4. Our objective is to characterize the most powerful flares observed in both bands and to constrain the energy released in the optical and X-ray, the geometry of the loops, and their time evolution. We aim to compare our results to existing studies of flares occurring in the Sun and stars at different ages. We selected bright X-ray/optical flares occurred in 12 known members of the Pleiades from their K2 and XMM-Newton light curves. The sample includes ten K-M stars, one F9 star, and one G8 star. Flare average properties were obtained from integrated analysis of the light curves during the flares. The time evolution of the plasma in the magnetic loops is constrained with time-resolved X-ray spectral analysis. Most of the flares studied in this work emitted more energy in optical than in X-rays, as in most solar flares, even if the Pleiades flares output a larger fraction of their total energy in X-rays than typical solar flares do. Additionally, the energy budget in the two bands is weakly correlated. We also found comparable flare duration in optical and X-rays and observed that rapidly rotating stars (e.g., with rotation period shorter than 0.5 days) preferentially host short flares. We estimated the slope of the cooling path of the flares in the log(EM)-versus-log(T) plane. The values we obtained are affected by large uncertainties, but their nominal values suggest that the flares analyzed in this paper are mainly due to single loops with no sustained heating occurring during the cooling phase. We also observed and analyzed oscillations with a period of 500 s during one of the flares.
Astronomy & Astrophysics, Volume 622, id.A210, 24 pp.Jan 2019
2018
- A multi-wavelength view of magnetic flaring from PMS stars
Abstract
Flares from the Sun and other stars are most prominently observed in the soft X-ray band. Most of the radiated energy, however, is released at optical/UV wavelengths. In spite of decades of investigation, the physics of flares is not fully understood. Even less is known about the powerful flares routinely observed from pre-main sequence stars, which might significantly influence the evolution of circumstellar disks. Observations of the NGC2264 star forming region were obtained in Dec. 2011, simultaneously with three telescopes, Chandra (X-rays), CoRoT (optical), and Spitzer (mIR), as part of the "Coordinated Synoptic Investigation of NGC2264" (CSI-NGC2264). Shorter Chandra and CoRoT observations were also obtained in March 2008. We analyzed the lightcurves to detect X-ray flares with an optical and/or mIR counterpart. Basic flare properties from the three datasets, such as emitted energies and peak luminosities, were then compared to constrain the spectral energy distribution of the flaring emission and the physical conditions of the emitting regions. Flares from stars with and without circumstellar disks were also compared to establish any difference that might be attributed to the presence of disks. Seventy-eight X-ray flares with an optical and/or mIR counterpart were detected. Their optical emission is found to correlate well with, and to be significantly larger than, the X-ray emission. The slopes of the correlations suggest that the difference becomes smaller for the most powerful flares. The mIR flare emission seems to be strongly affected by the presence of a circumstellar disk: flares from stars with disks have a stronger mIR emission with respect to stars without disks. This might be attributed to the reprocessing of the optical (and X-ray) flare emission by the inner circumstellar disk, providing evidence for flare-induced disk heating.
Astronomy & Astrophysics, Volume 620, id.A55, 34 pp.Jul 2018 - Low mass star formation and subclustering in the HII regions RCW 32, 33 and 27 of the Vela Molecular Ridge. A photometric diagnostics to identify M-type stars
Abstract
Most stars born in clusters and recent results suggest that star formation (SF) preferentially occurs in subclusters. Studying the morphology and SF history of young clusters is crucial to understanding early SF. We identify the embedded clusters of young stellar objects (YSOs) down to M stars, in the HII regions RCW33, RCW32 and RCW27 of the Vela Molecular Ridge. Our aim is to characterise their properties, such as morphology and extent of the clusters in the three HII regions, derive stellar ages and the connection of the SF history with the environment. Through public photometric surveys such as Gaia, VPHAS, 2MASS and Spitzer/GLIMPSE, we identify YSOs with IR, Halpha and UV excesses, as signature of circumstellar disks and accretion. In addition, we implement a method to distinguish M dwarfs and giants, by comparing the reddening derived in several optical/IR color-color diagrams, assuming suitable theoretical models. Since this diagnostic is sensitive to stellar gravity, the procedure allows us to identify pre-main sequence stars. We find a large population of YSOs showing signatures of circumstellar disks with or without accretion. In addition, with the new technique of M-type star selection, we find a rich population of young M stars with a spatial distribution strongly correlated to the more massive population. We find evidence of three young clusters, with different morphology. In addition, we identify field stars falling in the same region, by securely classifying them as giants and foreground MS stars. We identify the embedded population of YSOs, down to about 0.1 Msun, associated with the HII regions RCW33, RCW32 and RCW27 and the clusters Vela T2, Cr197 and Vela T1, respectively, showing very different morphologies. Our results suggest a decreasing SF rate in Vela T2 and triggered SF in Cr197 and Vela T1.
Astronomy & Astrophysics, Volume 617, id.A63, 20 pp.Jun 2018 - X-Ray Flare Oscillations Track Plasma Sloshing along Star-disk Magnetic Tubes in the Orion Star-forming Region
Abstract
Pulsing X-ray emission tracks the plasma “echo” traveling in an extremely long magnetic tube that flares in an Orion pre-main sequence (PMS) star. On the Sun, flares last from minutes to a few hours and the longest-lasting ones typically involve arcades of closed magnetic tubes. Long-lasting X-ray flares are observed in PMS stars. Large-amplitude (̃20%), long-period (̃3 hr) pulsations are detected in the light curve of day-long flares observed by the Advanced CCD Imaging Spectrometer on- board Chandra from PMS stars in the Orion cluster. Detailed hydrodynamic modeling of two flares observed on V772 Ori and OW Ori shows that these pulsations may track the sloshing of plasma along a single long magnetic tube, triggered by a sufficiently short (̃1 hr) heat pulse. These magnetic tubes are ≥20 solar radii long, enough to connect the star with the surrounding disk.
The Astrophysical Journal, Volume 856, Issue 1, article id. 51, 10 pp. (2018).Mar 2018
2017
- Fifteen years in the high-energy life of the solar-type star HD 81809. XMM-Newton observations of a stellar activity cycle
Abstract
Context. The modulation of the activity level of solar-like stars is commonly revealed by cyclic variations in their chromospheric indicators, such as the Ca ii H&K S-index, similarly to what is observed in our Sun. However, while the variation of solar activity is also reflected in the cyclical modulation of its coronal X-ray emission, similar behavior has only been discovered in a few stars other than the Sun. Aims: The data set of the long-term XMM-Newton monitoring program of HD 81809 is analyzed to study its X-ray cycle, investigate if the latter is related to the chromospheric cycle, infer the structure of the corona of HD 81809, and explore if the coronal activity of HD 81809 can be ascribed to phenomena similar to solar activity and, therefore, considered an extension of the solar case. Methods: We analyzed the observations of HD 81809 performed with XMM-Newton with a regular cadence of six months from 2001 to 2016, which represents one of the longest available observational baseline ( 15 yr) for a solar-like star with a well-studied chromospheric cycle (with a period of 8 yr). We investigated the modulation of coronal luminosity and temperature and its relation with the chromospheric cycle. We interpreted the data in terms of a mixture of solar-like coronal regions, adopting a method originally proposed to study the Sun as an X-ray star. Results: The observations show a well-defined regular cyclic modulation of the X-ray luminosity that reflects the activity level of HD 81809. The data covers approximately two cycles of coronal activity; the modulation has an amplitude of a factor of 5 (excluding evident flares, as in the June 2002 observation) and a period of 7.3 ± 1.5 yr, which is consistent with that of the chromospheric cycle. We demonstrate that the corona of HD 81809 can be interpreted as an extension of the solar case and can be modeled with a mixture of solar-like coronal regions along the whole cycle. The activity level is mainly determined by varying coverage of very bright active regions, similar to cores of active regions observed in the Sun. Evidence of unresolved significant flaring activity is present especially in the proximity of cycle maxima.
Astronomy & Astrophysics, Volume 605, id.A19, 10 pp.Sep 2017 - CSI 2264: Simultaneous optical and X-ray variability in pre-Main Sequence stars. I: Time resolved X-ray spectral analysis during optical dips and accretion bursts in stars with disks
Abstract
Pre-main sequence stars are variable sources. In stars with disks, this variability is related to the morphology of the inner circumstellar region (<0.1 AU) and that of the photosphere and corona, all impossible to be spatially resolved with present day techniques. This has been the main motivation for the Coordinated Synoptic Investigation of NGC 2264. In this paper, we focus on the stars with disks. We analyze the X-ray spectral properties extracted during optical bursts and dips in order to unveil the nature of these phenomena. We analyze simultaneous CoRoT and Chandra/ACIS-I observations to search for coherent optical and X-ray flux variability in stars with disks. Then, stars are analyzed in two different samples. In stars with variable extinction, we look for a simultaneous increase of optical extinction and X-ray absorption during the optical dips; in stars with accretion bursts, we search for soft X-ray emission and increasing X-ray absorption during the bursts. Results. We find evidence for coherent optical and X-ray flux variability among the stars with variable extinction. In 9/24 stars with optical dips, we observe a simultaneous increase of X-ray absorption and optical extinction. In seven dips, it is possible to calculate the NH/AV ratio in order to infer the composition of the obscuring material. In 5/20 stars with optical accretion bursts, we observe increasing soft X-ray emission during the bursts that we associate to the emission of accreting gas. It is not surprising that these properties are not observed in all the stars with dips and bursts, since favorable geometric configurations are required. The observed variable absorption during the dips is mainly due to dust-free material in accretion streams. In stars with accretion bursts, we observe on average a larger soft X-ray spectral component not observed in non accreting stars.
Astronomy & Astrophysics, Volume 602, id.A10, 79 pp.Jan 2017
2016
- A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function
Abstract
Context. NGC 6231 is a massive young star cluster, near the center of the Sco OB1 association. While its OB members are well studied, its low-mass population has received little attention. We present high-spatial resolution Chandra ACIS-I X-ray data, where we detect 1613 point X-ray sources. Aims: Our main aim is to clarify global properties of NGC 6231 down to low masses through a detailed membership assessment, and to study the cluster stars' spatial distribution, the origin of their X-ray emission, the cluster age and formation history, and initial mass function. Methods: We use X-ray data, complemented by optical and IR data, to establish cluster membership. The spatial distribution of different stellar subgroups also provides highly significant constraints on cluster membership, as does the distribution of X-ray hardness. We perform spectral modelling of group-stacked X-ray source spectra. Results: We find a large cluster population down to 0.3 M⊙ (complete to 1 M⊙), with minimal non-member contamination, with a definite age spread (1-8 Myr) for the low-mass PMS stars. We argue that low-mass cluster stars also constitute the majority of the few hundreds unidentified X-ray sources. We find mass segregation for the most massive stars. The fraction of circumstellar-disk bearing members is found to be 5%. Photoevaporation of disks under the action of massive stars is suggested by the spatial distribution of the IR-excess stars. We also find strong Hα emission in 9% of cluster PMS stars. The dependence of X-ray properties on mass, stellar structure, and age agrees with extrapolations based on other young clusters. The cluster initial mass function, computed over 2 dex in mass, has a slope Γ -1.14. The total mass of cluster members above 1 M⊙ is 2.28 × 103M⊙, and the inferred total mass is 4.38 × 103M⊙. We also study the peculiar, hard X-ray spectrum of the Wolf-Rayet star WR 79. Full Tables B.1 and B.2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/596/A82
Astronomy & Astrophysics, Volume 596, id.A82, 26 pp.Dec 2016 - The thin and medium filters of the EPIC camera on-board XMM-Newton: measured performance after more than 15 years of operation
Abstract
After more than 15 years of operation of the EPIC camera on board the XMM-Newton X-ray observatory, we have reviewed the status of its Thin and Medium filters. We have selected a set of Thin and Medium back-up filters among those still available in the EPIC consortium and have started a program to investigate their status by different laboratory measurements including: UV/VIS transmission, Raman scattering, X-Ray Photoelectron Spectroscopy, and Atomic Force Microscopy. Furthermore, we have investigated the status of the EPIC flight filters by performing an analysis of the optical loading in the PN offset maps to gauge variations in the optical and UV transmission. We both investigated repeated observations of single optically bright targets and performed a statistical analysis of the extent of loading versus visual magnitude at different epochs. We report the results of the measurements conducted up to now. Most notably, we find no evidence for change in the UV/VIS transmission of the back-up filters in ground tests spanning a 2 year period and we find no evidence for change in the optical transmission of the thin filter of the EPIC-pn camera from 2002 to 2012. We point out some lessons learned for the development and calibration programs of filters for X-ray detectors in future Astronomy missions.
Experimental Astronomy, Volume 42, Issue 2, pp.179-197Oct 2016 - Thermal Filters for the ATHENA X-IFU: Ongoing Activities Toward the Conceptual Design
Abstract
ATHENA is the L2 mission selected by ESA to pursue the science theme "Hot and Energetic Universe." One of the two focal plane instruments is the X-ray Integral Field Unit, an array of TES microcalorimeters operated at T < 100 mK. To allow the X-ray photons focused by the telescope to reach the detector, windows have to be opened on the cryostat thermal shields. X-ray transparent filters need to be mounted on these open windows to attenuate the IR radiation from warm surfaces, to attenuate RF electromagnetic interferences on TES sensors and SQUID electronics, and to protect the detector from contamination. This paper reviews the ongoing activities driving the design of the X-IFU thermal filters.
Journal of Low Temperature Physics, Volume 184, Issue 3-4, pp. 706-711Aug 2016 - Star-disk interaction in classical T Tauri stars revealed using wavelet analysis
Abstract
Context. The extension of the corona of classical T Tauri stars (CTTS) is is being widely discussed. The standard model of magnetic configuration of CTTS predicts that coronal magnetic flux tubes connect the stellar atmosphere to the inner region of the disk. However, differential rotation may disrupt these long loops. The results from hydrodynamic modeling of X-ray flares observed in CTTS that confirm the star-disk connection hypothesis are still controversial. Some authors suggest the presence of the accretion disk prevents the stellar corona extending beyond the co-rotation radius, while others are simply not confident with the methods used to derive loop lengths. Aims: We use independent procedures to determine the length of flaring loops in stars of the Orion Nebula Cluster, which has previously been analyzed using hydrodynamic models. Our aim is to disentangle the two scenarios that have been proposed. Methods: We present a different approach for determining the length of flaring loops that is based on the oscillatory nature of the loops after strong flares. We use wavelet tools to reveal oscillations during several flares. The subsequent analysis of these oscillations is based on the physics of coronal seismology. Results: Our results likely confirm the large extension of the corona of CTTS and the hypothesis of star-disk magnetic interaction in at least three CTTS of the Orion Nebula Cluster. Conclusions: Analyzing oscillations in flaring events is a powerful tool to determine the physical characteristics of magnetic loops in coronae in stars other than the Sun. The results presented in this work confirm the star-disk magnetic connection in CTTS.
Astronomy & Astrophysics, Volume 590, id.A7, 8 pp.May 2016 - Supersaturation and activity-rotation relation in PMS stars: the young cluster h Persei
Abstract
Context. Several studies showed that the magnetic activity of late-type main-sequence (MS) stars is characterized by different regimes and that their activity levels are well described by the Rossby number, Ro, defined as the ratio between the rotational period Prot and the convective turnover time. Very young pre-main-sequence (PMS) stars show, similarly to MS stars, intense magnetic activity. However, they do not show clear activity-rotation trends, and it still debated which stellar parameters determine their magnetic activity levels. Aims: To bridge the gap between MS and PMS stars, we studied the activity-rotation relation in the young cluster h Persei, a ~13 Myr old cluster, that contains both fast and slow rotators. The cluster members have ended their accretion phase and have developed a radiative core. It therefore offers us the opportunity of studying the activity level of intermediate-age PMS stars with different rotational velocities, excluding any interactions with the circumstellar environment. Methods: We constrained the magnetic activity levels of h Per members by measuring their X-ray emission from a Chandra observation, while rotational periods were obtained previously in the framework of the MONITOR project. By cross-correlating these data, we collected a final catalog of 414 h Per members with known rotational period, effective temperature, and mass. In 169 of these, X-ray emission has also been detected. Results: We found that h Per members with 1.0 M⊙<M⋆< 1.4 M⊙ display different activity regimes: fast rotators clearly show supersaturation, while slower rotators have activity levels compatible to the non-saturated regime. At 13 Myr, h Per is therefore the youngest cluster showing activity-rotation regimes analogous to those of MS stars, indicating that at this age, magnetic field production is most likely regulated by the αΩ type dynamo. Moreover, we observed that supersaturation is better described by Prot than Ro, and that the observed patterns are compatible with the hypothesis of centrifugal stripping. In this scenario we inferred that coronae can produce structures as large as ~2 R⋆ above the stellar surface. Tables 1 and 2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/589/A113
Astronomy & Astrophysics, Volume 589, id.A113, 14 pp.May 2016 - Results from DROXO. IV. EXTraS discovery of an X-ray flare from the Class I protostar candidate ISO-Oph 85
Abstract
X-ray emission from young stellar objects (YSOs) is a key ingredient in understanding star formation. For the early, protostellar (Class I) phase, a very limited (and controversial) quantity of X-ray results is available to date. Within the EXTraS (Exploring the X-ray Transient and variable Sky) project, we have discovered transient X-ray emission from a source whose counterpart is ISO-Oph 85, a strongly embedded YSO in the ρ Ophiuchi star-forming region. We extract an X-ray light curve for the flaring state, and determine the spectral parameters for the flare from XMM-Newton/EPIC data with a method based upon quantile analysis. We combine photometry from infrared to millimeter wavelengths from the literature with mid-IR Spitzer and unpublished submm Herschel photometry that we analysed for this work, and we describe the resulting spectral energy distribution (SED) with a set of precomputed models. The X-ray flare of ISO-Oph 85 lasted ~2500 s and is consistent with a highly-absorbed one-component thermal model (NH = 1.0-0.5+1.2 × 1023 cm-2 and kT= 1.15-0.65+2.35 keV). The X-ray luminosity during the flare is log LX [erg/s] = 31.1+2.0-1.2; during quiescence we set an upper limit of log LX [erg/s] < 29.5. We do not detect other flares from this source. The submillimeter fluxes suggest that the object is a Class I protostar. We caution, however, that the offset between the Herschel and optical/infrared position is larger than that for other YSOs in the region, leaving some doubt on this association. To the best of our knowledge, this is the first X-ray flare from a YSO that has been recognised as a candidate Class I protostar via the analysis of its complete SED, including the submm bands that are crucial for detecting the protostellar envelope. This work shows how the analysis of the whole SED is fundamental to the classification of YSOs, and how the X-ray source detection techniques we have developed can open a new era in time-resolved analysis of the X-ray emission from stars.
Astronomy & Astrophysics, Volume 587, id.A36, 9 pp.Mar 2016
2015
- X-Ray Emission from Massive Stars in Cyg OB2
Abstract
We report on the analysis of the Chandra-ACIS data of O, B, and WR stars in the young association Cyg OB2. X-ray spectra of 49 O-stars, 54 B-stars, and 3 WR-stars are analyzed and for the brighter sources, the epoch dependence of the X-ray fluxes is investigated. The O-stars in Cyg OB2 follow a well-defined scaling relation between their X-ray and bolometric luminosities: {log}\\frac{{L}{{X}}}{{L}{bol}}=-7.2+/- 0.2. This relation is in excellent agreement with the one previously derived for the Carina OB1 association. Except for the brightest O-star binaries, there is no general X-ray overluminosity due to colliding winds in O-star binaries. Roughly half of the known B-stars in the surveyed field are detected, but they fail to display a clear relationship between LX and Lbol. Out of the three WR stars in Cyg OB2, probably only WR 144 is itself responsible for the observed level of X-ray emission, at a very low {log}\\frac{{L}{{X}}}{{L}{bol}}=-8.8+/- 0.2. The X-ray emission of the other two WR-stars (WR 145 and 146) is most probably due to their O-type companion along with a moderate contribution from a wind-wind interaction zone.
The Astrophysical Journal Supplement Series, Volume 221, Issue 1, article id. 1, 20 pp. (2015).Nov 2015 - Coordinated X-Ray and Optical Observations of Star-Planet Interaction in HD 17156
Abstract
The large number of close-in Jupiter-size exoplanets prompts the question whether star-planet interaction (SPI) effects can be detected. We focused our attention on the system HD 17156, having a Jupiter-mass planet in a very eccentric orbit. Here we present results of the XMM-Newton observations and of a five month coordinated optical campaign with the HARPS-N spectrograph.10 We observed HD 17156 with XMM-Newton when the planet was approaching the apoastron and then at the following periastron passage, quasi-simultaneously with HARPS-N. We obtained a clear (≈ 5.5σ ) X-ray detection only at the periastron visit, accompanied by a significant increase of the {R}{HK}\prime chromospheric index. We discuss two possible scenarios for the activity enhancement: magnetic reconnection and flaring or accretion onto the star of material tidally stripped from the planet. In any case, this is possibly the first evidence of a magnetic SPI effect caught in action.
The Astrophysical Journal Letters, Volume 811, Issue 1, article id. L2, 5 pp. (2015).Sep 2015 - Spatial Distribution of X-Ray Emitting Ejecta in Tycho’s SNR: Indications of Shocked Titanium
Abstract
Young supernova remnants (SNRs) show characteristic ejecta-dominated X-ray emission that allows us to probe the products of explosive nucleosynthesis processes and to ascertain important information about the physics of supernova explosions. Hard X-ray observations have recently revealed the presence of the radioactive decay lines of 44Ti at ∼67.9 and ∼78.4 keV in Tycho’s SNR. Here, we analyze a set of XMM-Newton archive observations of Tycho’s SNR. We produce equivalent width (EW) maps of the Fe K and Ca xix emission lines and find indications for a stratification of the abundances of these elements and significant anisotropies. We then perform spatially resolved spectral analysis by identifying five different regions characterized by high/low values of the Fe K EW. We find that the spatial distribution of the Fe K emission is correlated with that of Cr xxii. We also detect the Ti K line complex in the spectra extracted from the two regions with the highest values of Fe and Cr EWs. The Ti line emission remains undetected in regions where Fe and Cr EWs are low. Our results indicate that the post-shock Ti is spatially colocated with other iron-peak nuclei in Tycho’s SNR, in agreement with the predictions of multi-D models of SNe Ia.
The Astrophysical Journal, Volume 805, Issue 2, article id. 120, 7 pp. (2015).Jun 2015 - FUV Variability of HD 189733. Is the Star Accreting Material From Its Hot Jupiter?
Abstract
Hot Jupiters are subject to strong irradiation from their host stars and, as a consequence, they do evaporate. They can also interact with the parent stars by means of tides and magnetic fields. Both phenomena have strong implications for the evolution of these systems. Here we present time-resolved spectroscopy of HD 189733 observed with the Cosmic Origins Spectrograph on board Hubble Space Telescope (HST). The star has been observed during five consecutive HST orbits, starting at a secondary transit of the planet (φ ∼ 0.50-0.63). Two main episodes of variability of ion lines of Si, C, N, and O are detected, with an increase of line fluxes. The Si iv lines show the highest degree of variability. The far-ultraviolet variability is a signature of enhanced activity in phase with the planet motion, occurring after the planet egress, as already observed three times in X-rays. With the support of MHD simulations, we propose the following interpretation: a stream of gas evaporating from the planet is actively and almost steadily accreting onto the stellar surface, impacting at 70{}^\circ -90{}^\circ ahead of the subplanetary point.
The Astrophysical Journal, Volume 805, Issue 1, article id. 52, 18 pp. (2015).May 2015
2014
- No X-rays from WASP-18. Implications for its age, activity, and the influence of its massive hot Jupiter
Abstract
About 20% of the >1000 known exoplanets are Jupiter analogs orbiting very close to their parent stars. It is still under debate to what detectable level such hot Jupiters possibly affect the activity of the host stars through tidal or magnetic star-planet interaction. In this paper we report on an 87 ks Chandra observation of the hot Jupiter hosting star WASP-18. This system is composed of an F6 type star and a hot Jupiter of mass 10.4 MJup orbiting in less than 20 hr around the parent star.On the basis of an isochrone fitting, WASP-18 is thought to be 600 Myr old and within the range of uncertainty of 0.5-2 Gyr.The star is not detected in X-rays down to a luminosity limit of 4 × 1026 erg/s, which is more than two orders of magnitude lower than expected for a star of this age and mass. This value proves an unusual lack of activity for a star with an estimated age around 600 Myr. We argue that the massive planet can play a crucial role in disrupting the stellar magnetic dynamo created within its thin convective layers. Other additional 212 X-ray sources are detected in the Chandra image. We list them and briefly discuss their nature. Full Table 2 is available in electronic form at http://www.aanda.org
Astronomy & Astrophysics, Volume 567, id.A128, 12 pp.Jul 2014 - Smooth X-ray variability from ρ Ophiuchi A+B. A strongly magnetized primary B2 star?
Abstract
X-rays from massive stars are ubiquitous yet not clearly understood. In an XMM-Newton observation devoted to observing the first site of star formation in the ρ Ophiuchi dark cloud, we detect smoothly variable X-ray emission from the B2IV+B2V system of ρ Ophiuchi. We tentatively assign the emission to the primary component. The light curve of the pn camera shows a first phase of low, almost steady rate, then a rise phase of duration of 10 ks, followed by a high rate phase. The variability is seen primarily in the band 1.0-8.0 keV while little variability is detected below 1 keV. The spectral analysis of the three phases reveals the presence of a hot component at 3.0 keV that adds up to two relatively cold components at 0.9 keV and 2.2 keV. We explain the smooth variability with the emergence of an extended active region on the surface of the primary star as being due to its fast rotation (vsin i ~ 315 km s-1). We estimate that the region has a diameter in the range 0.5-0.6 R∗. The hard X-ray emission and its variability hint at a magnetic origin, as suggested for a few other late-O through early-B type stars. We also discuss an alternative explanation based on the emergence from occultation of a young (5-10 Myr) low mass companion that is bright and hot in X-rays.
Astronomy & Astrophysics, Volume 567, id.L4, 4 pp.Jul 2014 - The Corona of HD 189733 and its X-Ray Activity
Abstract
Testing whether close-in massive exoplanets (hot Jupiters) can enhance the stellar activity in their host primary is crucial for the models of stellar and planetary evolution. Among systems with hot Jupiters, HD 189733 is one of the best studied because of its proximity, strong activity, and the presence of a transiting planet, which allows transmission spectroscopy and a measure of the planetary radius and its density. Here we report on the X-ray activity of the primary star, HD 189733 A, using a new XMM-Newton observation and a comparison with the previous X-ray observations. The spectrum in the quiescent intervals is described by two temperatures at 0.2 keV and 0.7 keV, while during the flares a third component at 0.9 keV is detected. With the analysis of the summed Reflection Grating Spectrometer spectra, we obtain estimates of the electron density in the range ne = (1.6-13) × 1010 cm-3, and thus the corona of HD 189733 A appears denser than the solar one. For the third time, we observe a large flare that occurred just after the eclipse of the planet. Together with the flares observed in 2009 and 2011, the events are restricted to a small planetary phase range of phi = 0.55-0.65. Although we do not find conclusive evidence of a significant excess of flares after the secondary transits, we suggest that the planet might trigger such flares when it passes close to the locally high magnetic field of the underlying star at particular combinations of stellar rotational phases and orbital planetary phases. For the most recent flares, a wavelet analysis of the light curve suggests a loop of length of four stellar radii at the location of the bright flare, and a local magnetic field of the order of 40-100 G, in agreement with the global field measured in other studies. The loop size suggests an interaction of magnetic nature between planet and star, separated by only ~8R *. The X-ray variability of HD 189733 A is larger than the variability of field stars and young Pleiades of similar spectral type and X-ray luminosity. We also detect the stellar companion (HD 189733 B, ~12'' from the primary star) in this XMM-Newton observation. Its very low X-ray luminosity (LX = 3.4 × 1026 erg s-1) confirms the old age of this star and of the binary system. The high activity of the primary star is best explained by a transfer of angular momentum from the planet to the star.
The Astrophysical Journal, Volume 785, Issue 2, article id. 145, 11 pp. (2014).Apr 2014
2013
- Erratum: “An X-Rays Survey of the Young Stellar Population of the Lynds 1641 and Iota Orionis Regions” (2013, ApJ, 768, 99)
Abstract
Not Available
The Astrophysical Journal, Volume 773, Issue 1, article id. 80, 2 pp. (2013).Aug 2013 - An X-Ray Survey of the Young Stellar Population of the Lynds 1641 and Iota Orionis Regions
Abstract
We present an XMM-Newton survey of the part of the Orion A cloud south of the Orion Nebula. This survey includes the Lynds 1641 (L1641) dark cloud, a region of the Orion A cloud with very few massive stars and hence a relatively low ambient UV flux, and the region around the O9 III star ι Orionis. In addition to proprietary data, we used archival XMM data of the Orion Nebula Cluster (ONC) to extend our analysis to a major fraction of the Orion A cloud. We have detected 1060 X-ray sources in L1641 and the ι Ori region. About 94% of the sources have Two Micron All Sky Survey and Spitzer counterparts, 204 and 23 being Class II and Class I or protostar objects, respectively. In addition, we have identified 489 X-ray sources as counterparts to Class III candidates, given they are bright in X-rays and appear as normal photospheres at mid-IR wavelengths. The remaining 205 X-ray sources are likely distant active galactic nuclei or other galactic sources not related to Orion A. We find that Class III candidates appear more concentrated in two main clusters in L1641. The first cluster of Class III stars is found toward the northern part of L1641, concentrated around ι Ori. The stars in this cluster are more evolved than those in the Orion Nebula. We estimate a distance of 300-320 pc for this cluster showing that it is in the foreground of the Orion A cloud. Another cluster rich in Class III stars is located in L1641 South and appears to be a slightly older cluster embedded in the Orion A cloud. Furthermore, other evolved Class III stars are found north of the ONC toward NGC 1977.
The Astrophysical Journal, Volume 768, Issue 2, article id. 99, 21 pp. (2013).May 2013 - Italian Astronomy at the Beginning of the 21st Century
Abstract
The end of the 20th century has seen a major reorganization of Italian astronomy. In about a decade, from 1999 to 2011, three successive laws have moved Italian astronomy from a system based on three major pillars (the twelve independent national astronomical observatories coordinated by the Public Education, University and Research Ministry through a consultative committee; the three CNR institutes devoted to astrophysics; and several university departments, either of astronomy or physics), to a single National Institute for Astrophysics (INAF). It includes all astronomical abservatories and the three CNR institutes, and collaborates with university groups operating within multi-thematic physics departments. This paper briefly describes the status, achievements and projects of Italian astrophysics, as well as some of the likely further changes coming up.
Organizations, People and Strategies in Astronomy Vol. 2, Edited by Andre Heck, Venngeist, Duttlenheim (2013) pp. 119-132Jan 2013
2012
- X-ray variability of pre-main-sequence stars. Toward an explanation of the different X-ray properties of classical and weak-line T Tauri stars
Abstract
Context. The first million years of the pre-main-sequence (PMS) evolution of low-mass stars are characterized by magnetospheric accretion, a circumstellar proto-planetary disk, and the processes leading to its dispersal. Among these, photo-evaporation caused by strong X-ray emission from the central star is probably significant. Several aspects of the X-ray emission from coronae and accretion shocks remain mysterious, e.g., whether and how much accretion affects coronal emission. Aims: We studied the X-ray variability of ~1 Myr old low-mass PMS stars as a function of timescale, stellar rotation, and stellar characteristics to gain insights into the working of PMS coronae, their X-ray emission, and the circumstellar environment in which they are immersed. Methods: We have exploited the ~850 ks long Chandra observation of the Orion Nebula Cluster obtained by the COUP collaboration in January 2003, and statistically analyzed the X-ray lightcurves of low-mass stars in several subsamples. Our main focus was to characterize the different X-ray behavior of stars with and without circumstellar accretion disks, and to infer the physical mechanism responsible for the observed variability. Results: Accreting stars (classical T Tauri stars, CTTSs) are found to be more variable than non-accreting stars (weak-line T Tauri stars, WTTSs) at all timescales and in all X-ray energy bands considered. Variability is seen to increase with timescale, up to the longest probed, ~10 days. Signatures of rotational modulation are observed for both CTTSs and WTTSs, and most clearly for CTTSs in the soft X-ray band. Lower mass stars are more variable than higher mass stars. Conclusions: We propose that the difference in variability between CTTSs and WTTSs may be explained assuming that the X-ray emission of CTTS is affected by time-variable absorption due to circumstellar structures, such as warps in the inner disk and/or accretion streams. This suggestion is appealing because, in the hypothesis that the coronae of CTTSs and WTTSs are similar, it may also explain why CTTSs have lower and more scattered X-ray emission levels compared to WTTSs. Appendices are available in electronic form at http://www.aanda.org
Astronomy & Astrophysics, Volume 548, id.A85, 16 pp.Dec 2012 - ORIGIN: metal creation and evolution from the cosmic dawn
Abstract
ORIGIN is a proposal for the M3 mission call of ESA aimed at the study of metal creation from the epoch of cosmic dawn. Using high-spectral resolution in the soft X-ray band, ORIGIN will be able to identify the physical conditions of all abundant elements between C and Ni to red- shifts of z = 10, and beyond. The mission will answer questions such as: When were the first metals created? How does the cosmic metal content evolve? Where do most of the metals reside in the Universe? What is the role of metals in structure formation and evolution? To reach out to the early Universe ORIGIN will use Gamma-Ray Bursts (GRBs) to study their local environments in their host galaxies. This requires the capability to slew the satellite in less than a minute to the GRB location. By studying the chemical composition and properties of clusters of galaxies we can extend the range of exploration to lower redshifts ( z ̃0.2). For this task we need a high-resolution spectral imaging instrument with a large field of view. Using the same instrument, we can also study the so far only partially detected baryons in the Warm-Hot Intergalactic Medium (WHIM). The less dense part of the WHIM will be studied using absorption lines at low redshift in the spectra for GRBs. The ORIGIN mission includes a Transient Event Detector (coded mask with a sensitivity of 0.4 photon/cm2/s in 10 s in the 5-150 keV band) to identify and localize 2000 GRBs over a five year mission, of which ̃65 GRBs have a redshift >7. The Cryogenic Imaging Spectrometer, with a spectral resolution of 2.5 eV, a field of view of 30 arcmin and large effective area below 1 keV has the sensitivity to study clusters up to a significant fraction of the virial radius and to map the denser parts of the WHIM (factor 30 higher than achievable with current instruments). The payload is complemented by a Burst InfraRed Telescope to enable onboard red-shift determination of GRBs (hence securing proper follow up of high-z bursts) and also probes the mildly ionized state of the gas. Fast repointing is achieved by a dedicated Controlled Momentum Gyro and a low background is achieved by the selected low Earth orbit.
Experimental Astronomy, Volume 34, Issue 2, pp.519-549Oct 2012 - Spectral classification and HR diagram of pre-main sequence stars in NGC 6530
Abstract
Context. Mechanisms involved in the star formation process and in particular the duration of the different phases of the cloud contraction are not yet fully understood. Photometric data alone suggest that objects coexist in the young cluster NGC 6530 with ages from ~1 Myr up to 10 Myr. Aims: We want to derive accurate stellar parameters and, in particular, stellar ages to be able to constrain a possible age spread in the star-forming region NGC 6530. Methods: We used low-resolution spectra taken with VLT/VIMOS and literature spectra of standard stars to derive spectral types of a subsample of 94 candidate members of this cluster. Results: We assign spectral types to 86 of the 88 confirmed cluster members and derive individual reddenings. Our data are better fitted by the anomalous reddening law with RV = 5. We confirm the presence of strong differential reddening in this region. We derive fundamental stellar parameters, such as effective temperatures, photospheric colors, luminosities, masses, and ages for 78 members, while for the remaining 8 YSOs we cannot determine the interstellar absorption, since they are likely accretors, and their V - I colors are bluer than their intrinsic colors. Conclusions: The cluster members studied in this work have masses between 0.4 and 4 M⊙ and ages between 1 - 2 Myr and 6 - 7 Myr. We find that the SE region is the most recent site of star formation, while the older YSOs are loosely clustered in the N and W regions. The presence of two distint generations of YSOs with different spatial distribution allows us to conclude that in this region there is an age spread of 6 - 7 Myr. This is consistent with the scenario of sequential star formation suggested in literature. Based on observations collected at the European Southern Observatory, Paranal, under program ID 077.C-0073B.Tables 1, 3, and 5 are available in electronic form at http://www.aanda.orgFITS files are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/546/A9
Astronomy & Astrophysics, Volume 546, id.A9, 22 pp.Oct 2012 - Chandra/ACIS-I Study of the X-Ray Properties of the NGC 6611 and M16 Stellar Populations
Abstract
Mechanisms regulating the origin of X-rays in young stellar objects and the correlation with their evolutionary stage are under debate. Studies of the X-ray properties in young clusters allow us to understand these mechanisms. One ideal target for this analysis is the Eagle Nebula (M16), with its central cluster NGC 6611. At 1750 pc from the Sun, it harbors 93 OB stars, together with a population of low-mass stars from embedded protostars to disk-less Class III objects, with age <=3 Myr. We study an archival 78 ks Chandra/ACIS-I observation of NGC 6611 and two new 80 ks observations of the outer region of M16, one centered on the Column V and the other on a region of the molecular cloud with ongoing star formation. We detect 1755 point sources with 1183 candidate cluster members (219 disk-bearing and 964 disk-less). We study the global X-ray properties of M16 and compare them with those of the Orion Nebula Cluster. We also compare the level of X-ray emission of Class II and Class III stars and analyze the X-ray spectral properties of OB stars. Our study supports the lower level of X-ray activity for the disk-bearing stars with respect to the disk-less members. The X-ray luminosity function (XLF) of M16 is similar to that of Orion, supporting the universality of the XLF in young clusters. Eighty-five percent of the O stars of NGC 6611 have been detected in X-rays. With only one possible exception, they show soft spectra with no hard components, indicating that mechanisms for the production of hard X-ray emission in O stars are not operating in NGC 6611.
The Astrophysical Journal, Volume 753, Issue 2, article id. 117, 16 pp. (2012).Jul 2012 - Star formation in the outer Galaxy: coronal properties of NGC 1893
Abstract
Context. The outer Galaxy, where the environmental conditions are different from the solar neighborhood, is a laboratory in which it is possible to investigate the dependence of star formation process on the environmental parameters. Aims: We investigate the X-ray properties of NGC 1893, a young cluster (~1-2 Myr) in the outer part of the Galaxy (galactic radius ≥11 kpc) where we expect differences in the disk evolution and in the mass distribution of the stars, to explore the X-ray emission of its members and compare it with that of young stars in star-forming regions near to the Sun. Methods: We analyze 5 deep Chandra ACIS-I observations with a total exposure time of 450 ks. Source events of the 1021 X-ray sources have been extracted with the IDL-based routine ACIS-Extract. Using spectral fitting and quantile analysis of X-ray spectra, we derive X-ray luminosities and compare the respective properties of Class II and Class III members. We also evaluate the variability of sources using the Kolmogorov-Smirnov test and identify flares in the lightcurves. Results: The X-ray luminosity of NGC 1893 X-ray members is in the range 1029.5-1031.5 erg s-1. Diskless stars are brighter in X-rays than disk-bearing stars, given the same bolometric luminosity. We found that 34% of the 1021 lightcurves appear variable and that they show 0.16 flare per source, on average. Comparing our results with those relative to the Orion Nebula Cluster, we find that, by accounting for observational biases, the X-ray properties of NGC 1893 and the Orion ones are very similar. Conclusions: The X-ray properties in NGC 1893 are not affected by the environment and the stellar population in the outer Galaxy may have the same coronal properties of nearby star-forming regions. The X-ray luminosity properties and the X-ray luminosity function appear to be universal and can therefore be used for estimating distances and for determining stellar properties. Full Tables 1 and 3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/539/A74
Astronomy & Astrophysics, Volume 539, id.A74, 14 pp.Mar 2012 - X-ray view of IC 348 in the light of an updated cluster census
Abstract
Context. IC 348 is a nearby ( ≈ 310 pc), young (~2-3 Myr) open cluster with >300 members identified from optical and infrared observations. It comprises young stellar objects in various evolutionary phases from protostars over disk-bearing to diskless pre-main sequence stars. This gives us the opportunity to study evolutionary effects in the high-energy emissions in a homogeneous environment. Aims: We study the properties of the coronae of the young low-mass stars in IC 348 combining X-ray and optical/infrared data. In particular, we intend to shed light on the dependence of X-ray luminosity and spectral hardness on evolutionary stage and on stellar parameters such as mass, effective temperature, and bolometric luminosity. Methods: The four existing Chandra observations of IC 348 were merged, thus providing a deeper and spatially more complete X-ray view than previous X-ray studies of the cluster. We have compiled a comprehensive catalog of IC 348 members taking into account recent updates to the cluster census. Our data collection comprises fundamental stellar parameters, infrared excess indicating the presence of disks, Hα emission as a tracer of chromospheric emission or accretion, and mass accretion rates. Results: We have detected 290 X-ray sources in four merged Chandra exposures, of which 187 are associated with known cluster members, which corresponds to a detection rate of ~60% for the cluster members of IC 348 that are identified in optical/infrared studies. According to the most recent spectral classification of IC 348 members, only four of the X-ray sources are brown dwarfs (spectral type M6 and later). The detection rate is highest for diskless Class III stars and increases with stellar mass. This may be explained with higher X-ray luminosities for higher mass and later evolutionary stage that is evident in the X-ray luminosity functions. In particular, we find that for the lowest examined masses (0.1-0.25 M⊙) there is a difference between the X-ray luminosity functions of accreting and non-accreting stars (classified on the basis of their Hα emission strength) as well as those of disk-bearing and diskless stars (classified on the basis of the slope of the spectral energy distribution). These differences disappear for higher masses. This is related to our finding that the Lx/Lbol ratio is non-constant across the mass/luminosity sequence of IC 348 with a decrease toward lower luminosity stars. Our analysis of an analogous stellar sample in the Orion Nebula Cluster suggests that the decline of Lx/Lbol for young stars at the low-mass end of the stellar sequence is likely universal. Full Tables 3 and 7 are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/537/A135
Astronomy & Astrophysics, Volume 537, id.A135, 15 pp.Jan 2012
2011
- Star formation in the outer Galaxy: membership and fundamental parameters of the young open cluster NGC 1893
Abstract
Context. Different environmental conditions can play a crucial role in determining final products of the star formation process, and in this context, less favorable activities of star formation are expected in the external regions of our Galaxy. Aims: We studied the properties of the young open cluster NGC 1893 located about 12 Kpc from the galactic center, to investigate how different physical conditions can affect the process of star formation. Methods: By adopting a multiwavelength approach, we compiled a catalog extending from X-rays to NIR data to derive the cluster membership. In addition, optical and NIR photometric properties are used to evaluate the cluster parameters. Results: We find 415 diskless candidate members and 1061 young stellar objects with a circumstellar disk or class II candidate members, 125 of which are also Hα emitters. Considering the diskless candidate members, we find that the cluster distance is 3.6 ± 0.2 kpc and the mean interstellar reddening is E(B - V) = 0.6 ± 0.1 with evidence of differential reddening in the whole surveyed region. Conclusions: NGC 1893 contains a conspicuous population of pre-main sequence stars, together with the well-studied main sequence cluster population. We found a disk fraction of about 70% similar to the one found in clusters of similar age in the solar neighbor and then, despite expected unfavorable conditions for star formation, we conclude that very rich young clusters can also form in the outer regions of our Galaxy. Full Tables 5-8 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/527/A77
Astronomy & Astrophysics, Volume 527, id.A77, 19 pp.Mar 2011
2010
- Pre-main sequence stars with disks in the Eagle Nebula observed in scattered light
Abstract
Context. NGC 6611 and its parental cloud, the Eagle Nebula (M 16), are well-studied star-forming regions, thanks to their large content of both OB stars and stars with disks and the observed ongoing star formation. In our previous studies of the Eagle Nebula, we identified 834 disk-bearing stars associated with the cloud, after detecting their excesses in NIR bands from J band to 8.0 μ m. Aims: In this paper, we study in detail the nature of a subsample of disk-bearing stars that show peculiar characteristics. They appear older than the other members in the V vs. V-I diagram, and/or they have one or more IRAC colors at pure photospheric values, despite showing NIR excesses, when optical and infrared colors are compared. Methods: We confirm the membership of these stars to M 16 by a spectroscopic analysis. The physical properties of these stars with disks are studied by comparing their spectral energy distributions (SEDs) with the SEDs predicted by models of T Tauri stars with disks and envelopes. Results: We show that the age of these stars estimated from the V vs. V-I diagram is unreliable since their V-I colors are altered by the light scattered by the disk into the line of sight. Only in a few cases their SEDs are compatible with models with excesses in V band caused by optical veiling. Candidate members with disks and photospheric IRAC colors are selected by the used NIR disk diagnostic, which is sensitive to moderate excesses, such as those produced by disks with low masses. In 1/3 of these cases, scattering of stellar flux by the disks can also be invoked. Conclusions: The photospheric light scattered by the disk grains into the line of sight can affect the derivation of physical parameters of Class II stars from photometric optical and NIR data. Besides, the disks diagnostic we defined are useful for selecting stars with disks, even those with moderate excesses or whose optical colors are altered by veiling or photospheric scattered light. Table with the data of the stars is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/521/A18
Astronomy and Astrophysics, Volume 521, id.A18, 9 pp.Oct 2010 - Chronology of star formation and disk evolution in the Eagle Nebula
Abstract
Context. Massive star-forming regions are characterized by intense ionizing fluxes, strong stellar winds and, occasionally, supernovae explosions, all of which have important effects on the surrounding media, on the star-formation process and on the evolution of young stars and their circumstellar disks. We present a multiband study of the massive young cluster NGC 6611 and its parental cloud (the Eagle Nebula) with the aim of studying how OB stars affect the early stellar evolution and the formation of other stars. Aims: We search for evidence of triggering of star formation by the massive stars inside NGC 6611 on a large spatial scale (~10 parsec) and ongoing disk photoevaporation in NGC 6611 and how its efficiency depends on the mass of the central stars. Methods: We assemble a multiband catalog of the Eagle Nebula with photometric data, ranging from B band to 8.0 μm, and X-ray data obtained with two new and one archival Chandra/ACIS-I observation. We select the stars with disks from infrared photometry and disk-less ones from X-ray emission, which are associated both with NGC 6611 and the outer region of the Eagle Nebula. We study induced photoevaporation searching for the spatial variation of disk frequency for distinct stellar mass ranges. The triggering of star formation by OB stars has been investigated by deriving the history of star formation across the nebula. Results: We find evidence of sequential star formation in the Eagle Nebula going from the southeast (2.6 Myears) to the northwest (0.3 Myears), with the median age of NGC 6611 members ~1 Myear. In NGC 6611, we observe a drop of the disk frequency close to massive stars (up to an average distance of 1 parsec), without observable effects at larger distances. Furthermore, disks are more frequent around low-mass stars (≤ 1 M⊙) than around high-mass stars, regardless of the distance from OB stars. Conclusions: The star-formation chronology we find in the Eagle Nebula does not support the hypothesis of a large-scale process triggered by OB stars in NGC 6611. Instead, we speculate that it was triggered by the encounter (about 3 Myears ago) with a giant molecular shell created by supernovae explosions about 6 Myears ago. We find evidence of disk photoevaporation close to OB stars, where disks are heated by incident extreme ultraviolet (EUV) radiation. No effects are observed at large distances from OB stars, where photoevaporation is induced by the far ultraviolet (FUV) radiation, and long timescales are usually required to completely dissipate the disks.
Astronomy and Astrophysics, Volume 521, id.A61, 17 pp.Oct 2010 - Results from DROXO. III. Observation, source list, and X-ray properties of sources detected in the “Deep Rho Ophiuchi XMM-Newton Observation”
Abstract
Context. X-rays from very young stars are powerful probes to investigate the mechanisms at work in the very first stages of the star formation and the origin of X-ray emission in very young stars. Aims: We present results from a 500 ks long observation of the Rho Ophiuchi cloud with a XMM-Newton large program named DROXO, aiming at studying the X-ray emission of deeply embedded young stellar objects (YSOs). Methods: The data acquired during the DROXO program were reduced with SAS software, and filtered in time and energy to improve the signal to noise of detected sources; light curves and spectra were obtained. Results: We detected 111 sources, 61 of them associated with ρ Ophiuchi YSOs as identified from infrared observations with ISOCAM. Specifically, we detected 9 out of 11 Class I objects, 31 out of 48 Class II and 15 out 16 Class III objects. Six objects out of 21 classified Class III candidates are also detected. At the same time we suggest that 15 Class III candidates that remain undetected at logLx [erg/s] < 28.3 are not related to the cloud population. The global detection rate is ~64%. We have achieved a flux sensitivity of ~5 × 10-15 erg s-1 cm-2. The L_X to L_bol ratio shows saturation at a value of ~-3.5 for stars with T_eff ≤ 5000 K or 0.7 M_⊙ as observed in the Orion Nebula. The plasma temperatures and the spectrum absorption show a decline with YSO class, with Class I YSOs being hotter and more absorbed than Class II and III YSOs. In one star (GY 266) with infrared counterpart in 2MASS and Spitzer catalogs we have detected a soft excess in the X-ray spectrum, which is best fitted by a cold thermal component less absorbed than the main thermal component of the plasma. This soft component hints at plasma heated by shocks due to jets outside the dense circumstellar material.
Astronomy and Astrophysics, Volume 519, id.A34, 27 pp.Sep 2010
2009
- X-ray optical depth diagnostics of T Tauri accretion shocks
Abstract
Context: In classical T Tauri stars, X-rays are produced by two plasma components: a hot low-density plasma, with frequent flaring activity, and a high-density lower temperature plasma. The former is coronal plasma related to the stellar magnetic activity. The latter component, never observed in non-accreting stars, could be plasma heated by the shock formed by the accretion process. However its nature is still being debated. Aims: Our aim is to probe the soft X-ray emission from the high-density plasma component in classical T Tauri stars to check whether this plasma is heated in the accretion shock or whether it is coronal plasma. Methods: High-resolution X-ray spectroscopy allows us to measure individual line fluxes. We analyze X-ray spectra of the classical T Tauri stars MP Muscae and TW Hydrae. Our aim is to evaluate line ratios to search for optical depth effects, which are expected in the accretion-driven scenario. We also derive the plasma emission measure distributions EMD, to investigate whether and how the EMD of accreting and non accreting young stars differ. The results are compared to those obtained for the non-accreting weak-line T Tauri star TWA 5. Results: We find evidence of resonance scattering in the strongest lines of MP Mus, supporting the idea that soft X-rays are produced by plasma heated in the accretion shock. We also find that the EMD of MP Mus has two peaks: a cool peak at temperatures expected for plasma heated in the accretion shock, and a hot peak typical of coronal plasma. The shape of the EMD of MP Mus appears to be the superposition of the EMD of a pure coronal source, like TWA 5, and an EMD alike that of TW Hya, which is instead dominated by shock-heated plasma.
Astronomy and Astrophysics, Volume 507, Issue 2, 2009, pp.939-948Nov 2009 - Results from DROXO. II. [Ne II] and X-ray emission from ρ Ophiuchi young stellar objects
Abstract
Context: The infrared [Ne II] and [Ne III] fine structure lines at 12.81 μm and 15.55 μm have recently been theoretically predicted to trace the circumstellar disk gas subject to X-ray heating and ionization. Aims: We observationally investigate the origin of the neon fine structure line emission by comparing observations with models of X-ray irradiated disks and by searching for empirical correlations between the line luminosities and stellar and circumstellar parameters. Methods: We select a sample of 28 young stellar objects in the ρ Ophiuchi star formation region for which good quality infrared spectra and X-ray data have been obtained, the former with the Spitzer/IRS and the latter with the deep rho Ophiuchi XMM-Newton observation (DROXO). We measure neon line fluxes and X-ray luminosities; we complement these data with stellar/circumstellar parameters obtained by fitting the spectral energy distributions of our objects (from optical to millimeter wavelengths) with star/disk/envelope models. Results: We detect the [Ne II] and the [Ne III] lines in 10 and 1 cases, respectively. Line luminosities show no correlation with X-ray emission. The luminosity of the [Ne II] line for one star, and that of both the [Ne II] and [Ne III] lines for a second star, match the predictions of published models of X-ray irradiated disks; for the remaining 8 objects the [Ne II] emission is 1-3 dex higher than predicted on the basis of their L_X. However, the stellar/circumstellar characteristics assumed in published models do not match those of most of the stars in our sample. Class I objects show significantly stronger [Ne II] lines than Class II and Class III ones. A correlation is moreover found between the [Ne II] line emission and the disk mass accretion rates estimated from the spectral energy distributions. This might point toward a role of accretion-generated UV emission in the generation of the line or to other mechanisms related to mass inflows from circumstellar disks and envelopes and/or to the associated mass outflows (winds and jets). Conclusions: The X-ray luminosity is clearly not the only parameter that determines the [Ne II] emission. For more exacting tests of X-ray irradiated disk models, these must be computed for the stellar and circumstellar characteristics of the observed objects. Explaining the strong [Ne II] emission of Class I objects likely requires the inclusion in the models of additional physical components such as the envelope, inflows, and outflows. Appendix A is only available in electronic form at http://www.aanda.org
Astronomy and Astrophysics, Volume 505, Issue 2, 2009, pp.695-706Oct 2009 - EDGE: Explorer of diffuse emission and gamma-ray burst explosions
Abstract
How structures of various scales formed and evolved from the early Universe up to present time is a fundamental question of astrophysical cosmology. EDGE (Piro et al., 2007) will trace the cosmic history of the baryons from the early generations of massive stars by Gamma-Ray Burst (GRB) explosions, through the period of galaxy cluster formation, down to the very low redshift Universe, when between a third and one half of the baryons are expected to reside in cosmic filaments undergoing gravitational collapse by dark matter (the so-called warm hot intragalactic medium). In addition EDGE, with its unprecedented capabilities, will provide key results in many important fields. These scientific goals are feasible with a medium class mission using existing technology combined with innovative instrumental and observational capabilities by: (a) observing with fast reaction Gamma-Ray Bursts with a high spectral resolution. This enables the study of their star-forming and host galaxy environments and the use of GRBs as back lights of large scale cosmological structures; (b) observing and surveying extended sources (galaxy clusters, WHIM) with high sensitivity using two wide field of view X-ray telescopes (one with a high angular resolution and the other with a high spectral resolution). The mission concept includes four main instruments: a Wide-field Spectrometer (0.1-2.2 eV) with excellent energy resolution (3 eV at 0.6 keV), a Wide-Field Imager (0.3-6 keV) with high angular resolution (HPD = 15”) constant over the full 1.4 degree field of view, and a Wide Field Monitor (8-200 keV) with a FOV of ¼ of the sky, which will trigger the fast repointing to the GRB. Extension of its energy response up to 1 MeV will be achieved with a GRB detector with no imaging capability. This mission is proposed to ESA as part of the Cosmic Vision call. We will outline the science drivers and describe in more detail the payload of this mission.
Experimental Astronomy, Volume 23, Issue 1, pp.67-89Mar 2009 - XEUS: the physics of the hot evolving universe
Abstract
This paper describes the next generation X-ray observatory XEUS which has been submitted to the European Space Agency in the framework of the Cosmic Vision 2015-2025 competition and has been selected for an assessment study. The paper summarizes the scientific goals and instrumental concepts of the proposed X-ray telescope with 5 m2 effective area and angular resolution better than 5 arc sec.
Experimental Astronomy, Volume 23, Issue 1, pp.139-168Mar 2009 - Correlation between the spatial distribution of circumstellar disks and massive stars in the young open cluster NGC 6611. II. Cluster members selected with Spitzer/IRAC
Abstract
Context: The observations of the proplyds in the Orion Nebula Cluster, exhibiting clear evidence of ongoing photoevaporation, have provided clear proof of the role of externally induced photoevaporation in the evolution of circumstellar disks. NGC 6611 is an open cluster suitable for study of disk photoevaporation, due to its significant population of massive members and stars with disk. In a previous paper, we obtained evidence of the influence of the strong UV field generated by the massive cluster members on the evolution of disks around low-mass Pre-Main Sequence members. Our study was based on a multiband BVIJHK and X-ray catalog compiled for the purpose of selecting cluster members with and without disk. Aims: We attempt to complete the list of candidate cluster members, using data at longer wavelengths obtained with Spitzer/IRAC, and we reinvestigate the effects of UV radiation on the evolution of disks in NGC 6611. Methods: In a field of view of 33'×34' centered on the cluster, we select the candidate members with disks of NGC 6611 using IRAC color-color diagrams and suitable reddening-free color indices. Using the X-ray data to select Class III cluster members, we also estimate disks frequency relative to the intensity of the incident radiation emitted by massive members. Results: We identify 458 candidate members with circumstellar disks, among which 146 had not been discovered previously. By comparing all color indices used to select cluster members with disk, we claim that these indices measure the excess of radiation due to the emission of the same physical region of the disk (i.e. the inner rim at the dust sublimation radius). Our new results confirm that UV radiation from massive stars affects the evolution of nearby circumstellar disks. Optical-infrared catalog of the candidate members of the open cluster NGC 6611 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/496/453
Astronomy and Astrophysics, Volume 496, Issue 2, 2009, pp.453-463Mar 2009 - Results from DROXO. I. The variability of fluorescent Fe 6.4 keV emission in the young star Elias 29. High-energy electrons in the star’s accretion tubes?
Abstract
Not Available
Astronomy and Astrophysics, Volume 495, Issue 3, 2009, pp.899-900Mar 2009
2008
- An X-ray survey of low-mass stars in Trumpler 16 with Chandra
Abstract
Aims: We identify and characterize low-mass stars in the 3 Myr old Trumpler 16 region by means of a deep Chandra X-ray observation, and study their optical and near-IR properties. We compare the X-ray activity of Trumpler 16 stars with the known characteristics of Orion and Cygnus OB2 stars. Methods: We analyzed a 88.4 ks Chandra ACIS-I observation pointed at the center of Trumpler 16. Because of diffuse X-ray emission, source detection was performed using the PWDetect code for two different energy ranges: 0.5-8.0 keV and 0.9-8.0 keV. Results were merged into a single final list. We positionally correlated X-ray sources with optical and 2MASS catalogs. Source events were extracted with the IDL-based routine ACIS-Extract. X-ray variability was characterized using the Kolmogorov-Smirnov test and spectra were fitted by using XSPEC. The X-ray spectra of early-type, massive stars were analyzed individually. Results: Our list of X-ray sources consists of 1035 entries, 660 of which have near-IR counterparts and are probably associated with Trumpler 16 members. From near-IR, color-color, and color-magnitude diagrams we compute individual masses of stars and their Av values. The cluster median extinction is Av = 3.6 mag, while OB-type stars appear less absorbed, having Av = 2.0 mag. About 15% of the near-IR counterparts show disk-induced excesses. X-ray variability is found in 77 sources, and typical X-ray spectral parameters are N_H˜ 5.37 × 1021 cm-2 and kT˜ 1.95 keV. The OB stars appear, softer with a median kT˜ 0.65 keV. The median X-ray luminosity is 6.3 × 1030 {erg s-1}, while variable sources show a larger median Lx value of 13 × 1030 {erg s-1}. OB-stars have an even higher median Lx of 80 × 1030 {erg s-1}, about 10 times that of the low-mass stars. Conclusions: The Trumpler 16 region has a very rich population of low-mass X-ray emitting stars. A large fraction of its circumstellar disks have survived the intense radiation field of its massive stars. Stars with masses 1.5-2.5 M_⊙ display X-ray activity similar to the Cyg OB2 stars, but much less intense than observed for Orion nebula cluster members. Full Tables [see full textsee full text]-[see full textsee full text] are only available in electronic form at http://www.aanda.org
Astronomy and Astrophysics, Volume 490, Issue 3, 2008, pp.1055-1070Nov 2008 - The X-ray cycle in the solar-type star HD 81809. XMM-Newton observations and implications for the coronal structure
Abstract
Context: The 11-yr cycle is the best known manifestation of the Sun's activity. While chromospheric cycles have been studied in a number of solar-like stars, very little is known about how these are reflected in the cyclical behavior of the coronal X-ray emission in stars other than the Sun. Aims: Our long-term XMM-Newton program of long-term monitoring of a solar-like star with a well-studied chromospheric cycle, HD 81809, aims to study whether an X-ray cycle is present, along with studying its characteristics and its relation to the chromospheric cycle. Methods: Regular observations of HD 81809 were performed with XMM-Newton, spaced by 6 months from 2001 to 2007. We studied the variations in the resulting coronal luminosity and temperature, and compared them with the chromospheric Ca ii variations. We also modeled the observations in terms of a mixture of active regions, using a methodology originally developed to study the solar corona. Results: Our observations show a well-defined cycle with an amplitude exceeding 1 dex and an average luminosity approximately one order of magnitude higher than in the Sun. The behavior of the corona of HD 81809 can be modeled well in terms of varying coverage of solar-like active regions, with a larger coverage than for the Sun, showing it to be compatible with a simple extension of the solar case.
Astronomy and Astrophysics, Volume 490, Issue 3, 2008, pp.1121-1126Nov 2008 - Circumstellar disks in the outer Galaxy: the star-forming region NGC 1893
Abstract
Context: It is still debated whether star formation process depends on environment. In particular it is yet unclear whether star formation in the outer Galaxy, where the environmental conditions are, theoretically, less conducive, occurs in the same way as in the inner Galaxy. Aims: We investigate the population of NGC 1893, a young cluster ( 3{-}4 Myr) in the outer part of the Galaxy (RG ≥ 11 kpc), to explore the effects of environmental conditions on star forming regions. Methods: We present infrared observations acquired using the IRAC camera onboard the Spitzer Space Telescope and analyze the color-color diagrams to establish the membership of stars with excesses. We also merge this information with that obtained from Chandra ACIS-I observations, to identify the Class III population. Results: We find that the cluster is very rich, with 242 PMS Classical T Tauri stars and 7 Class 0/I stars. We identify 110 Class III candidate cluster members in the ACIS-I field of view. We estimate a disk fraction for NGC 1893 of about 67%, similar to fractions calculated for nearby star forming regions of the same age. Conclusions: Although environmental conditions are unfavorable, star formation can clearly be very successful in the outer Galaxy, allowing creation of a very rich cluster like NGC 1893.
Astronomy and Astrophysics, Volume 488, Issue 1, 2008, pp.211-218Sep 2008 - Results and perspectives of young stellar object long look programs
Abstract
Both Chandra and XMM-Newton have performed long look programs for studying the YSO physics. I will discuss recent results on the controversial issue of Class 0 YSO X-ray emission, the observational evidence of magnetic funnels interconnecting the YSO with its circumstellar disk and the Fe 6.4 keV fluorescent line emission and its origin. While recent results of the XMM-Newton DROXO program challenge the ``standard" interpretation of the Fe 6.4 kev line origin as due to photoionized fluorescing disk material, the discovery of X-ray excited Ne 12.81 μm line is a clear evidence of the interaction between X-rays and disk material. Future long look observations with XMM-Newton are required to clarify the X-ray effects on YSO disk.
Astronomische Nachrichten, Vol.329, Issue 2, p.214Feb 2008
2007
- Results from Droxo. I. The variability of fluorescent Fe 6.4 keV emission in the young star Elias 29. High-energy electrons in the star’s accretion tubes?
Abstract
Aims:We study the variability of the Fe 6.4 keV emission line from the Class I young stellar object Elias 29 in the ρ Oph cloud. Methods: We analysed the data from Elias 29 collected by XMM-Newton during a nine-day, nearly continuous observation of the ρ Oph star-forming region (the Deep Rho-Oph X-ray Observation, named droxo). The data were subdivided into six homogeneous time intervals, and the six resulting spectra were individually analysed. Results: We detect significant variability in the equivalent width of the Fe 6.4 keV emission line from Elias 29. The 6.4 keV line is absent during the first time interval of observation and appears at its maximum strength during the second time interval (90 ks after Elias 29 undergoes a strong flare). The X-ray thermal emission is unchanged between the two observation segments, while line variability is present at a 99.9% confidence level. Given the significant line variability in the absence of variations in the X-ray ionising continuum and the weakness of the photoionising continuum from the star's thermal X-ray emission, we suggest that the fluorescence may be induced by collisional ionisation from an (unseen) population of non-thermal electrons. We speculate on the possibility that the electrons are accelerated in a reconnection event of a magnetically confined accretion loop, connecting the young star to its circumstellar disk.
Astronomy and Astrophysics, Volume 475, Issue 3, December I 2007, pp.891-900Dec 2007 - X-ray flaring from the young stars in Cygnus OB2
Abstract
Aims:We characterize individual and ensemble properties of X-ray flares from stars in the Cygnus OB2 and ONC star-forming regions. Methods: We analyzed X-ray lightcurves of 1003 Cygnus OB2 sources observed with Chandra for 100 ks and of 1616 ONC sources detected in the “Chandra Orion Ultra-deep Project” 850 ks observation. We employed a binning-free maximum likelihood method to segment the light-curves into intervals of constants signal and identified flares on the basis of both the amplitude and the time-derivative of the source luminosity. We then derived and compared the flare frequency and energy distribution of Cygnus OB2 and ONC sources. The effect of the length of the observation on these results was investigated by repeating the statistical analysis on five 100 ks-long segments extracted from the ONC data. Results: We detected 147 and 954 flares from the Cygnus OB2 and ONC sources, respectively. The flares in Cygnus OB2 have decay times ranging from ⪉0.5 to about 10 h. The flare energy distributions of all considered flare samples are described at high energies well by a power law with index α=-(2.1±0.1). At low energies, the distributions flatten, probably because of detection incompleteness. We derived average flare frequencies as a function of flare energy. The flare frequency is seen to depend on the source's intrinsic X-ray luminosity, but its determination is affected by the length of the observation. The slope of the high-energy tail of the energy distribution is, however, affected little. A comparison of Cygnus OB2 and ONC sources, accounting for observational biases, shows that the two populations, known to have similar X-ray emission levels, have very similar flare activity. Conclusions: Studies of flare activity are only comparable if performed consistently and taking the observation length into account. Flaring activity does not vary appreciably between the age of the ONC (~1 Myr) and that of Cygnus OB2 (~2 Myr). The slope of the distribution of flare energies is consistent with the micro-flare explanation of the coronal heating. Full Table 1 is only available in electronic form at http://www.aanda.org
Astronomy and Astrophysics, Volume 474, Issue 2, November I 2007, pp.495-504Nov 2007 - Coronal Abundances in Orion Nebula Cluster Stars
Abstract
Following the Chandra Orion Ultradeep Project (COUP) observation, we have studied the chemical composition of the hot plasma in a sample of 146 X-ray-bright pre-main-sequence stars in the Orion Nebula Cluster (ONC). We report measurements of individual element abundances for a subsample of 86 slightly absorbed and bright X-ray sources, using low-resolution X-ray spectra obtained from the Chandra ACIS instrument. The X-ray emission originates from a plasma with temperatures and elemental abundances very similar to those of active coronae in older stars. A clear pattern of abundances versus first ionization potential (FIP) is evident if solar photospheric abundances are assumed as reference. The results are validated by extensive simulations. The observed abundance distributions are compatible with a single pattern of abundances for all stars, although a weak dependence on flare loop size may be present. The abundance of calcium is the only one which appears to vary substantially between stars, but this quantity is affected by relatively large uncertainties. The ensemble properties of the X-ray-bright COUP sources confirm that the iron in the emitting plasma is underabundant with respect to both the solar composition and the average stellar photospheric values. Comparison of the present plasma abundances with those of the stellar photospheres and those of the gaseous component of the nebula indicates a good agreement for all the other elements with available measurements, and in particular for the high-FIP elements (Ne, Ar, O, and S) and for the low-FIP element Si. We conclude that there is evidence of a significant chemical fractionation effect only for iron, which appears to be depleted by a factor 1.5-3 with respect to the stellar composition.
The Astrophysical Journal, Volume 660, Issue 2, pp. 1462-1479.May 2007 - Unveiling the Cygnus OB2 stellar population with Chandra
Abstract
Aims:The aim of this work is to identify the so far unknown low mass stellar population of the ~2 Myr old Cygnus OB2 star forming region, and to investigate the X-ray and near-IR stellar properties of its members. Methods: We analyzed a 97.7 ks Chandra ACIS-I observation pointed at the core of the Cygnus OB2 region. Sources were detected using the PWDetect code and were positionally correlated with optical and near-IR catalogs from the literature. Source events were extracted with the Acis Extract package. X-ray variability was characterized through the Kolmogorov-Smirnov test and spectra were fitted using absorbed thermal plasma models. Results: We detected 1003 X-ray sources. Of these, 775 have near-IR counterparts and are expected to be almost all associated with Cygnus OB2 members. From near-IR color-color and color-magnitude diagrams we estimate a typical absorption toward Cygnus OB2 of Av ≈ 7.0 mag. Although the region is young, very few stars (~4.4%) show disk-induced excesses in the near-IR. X-ray variability is detected in ~13% of the sources, but this fraction increases, up to 50%, with increasing source statistics. Flares account for at least 60% of the variability. Despite being generally bright, all but 2 of the 26 detected O- that early B-type stars are not significantly variable. Typical X-ray spectral parameters are log N_H˜ 22.25 (cm-2) and kT˜ 1.35 keV with 1σ dispersion of 0.2 dex and 0.4 keV, respectively. Variable and flaring sources have harder spectra with median kT= 3.3 and 3.8 keV, respectively. OB stars are typically softer (kT ˜ 0.75 keV). X-ray luminosities range between 1030 and 1031 erg s-1 for intermediate- and low-mass stars, and 2.5×1030 and between 6.3×1033 erg s-1 for OB stars. Conclusions: .The Cygnus OB2 region has a very rich population of low-mass X-ray emitting stars. Circumstellar disks seem to be very scarce. X-ray variability is related to the magnetic activity of low-mass stars (M/M_⊙ ˜0.5 to 3.0) display X-ray activity levels comparable to those of Orion Nebular Cluster (ONC) sources in the same mass range. Tables 1-3 are only available in electronic form at http://www.aanda.org
Astronomy and Astrophysics, Volume 464, Issue 1, March II 2007, pp.211-227Mar 2007 - The stellar content of the XMM-Newton bright serendipitous survey
Abstract
Context: The comparison of observed counts in a given sky direction with predictions by Galactic models yields constraints on the spatial distribution and the stellar birthrate of young stellar populations. The XMM-Newton Bright Serendipitous Survey (XBSS) is an unbiased survey that includes a total of 58 stellar sources selected in the 0.5-4.5 keV energy band, having a limiting sensitivity of 10-2 cnt s-1 and covering an area of 28.10 sq deg. Aims: We present the results of analysing the stellar content of the XBSS so as to understand the recent star formation history of the Galaxy in the vicinity of the Sun. Methods: We compared the observations with the predictions obtained with XCOUNT, a model of the stellar X-ray content of the Galaxy. The model predicts the number and properties of the stars to be observed in terms of magnitude, colour, population and f_x/fv ratio distributions of the coronal sources detected with a given instrument and sensitivity in a specific sky direction. Results: As in other shallow surveys, we observe an excess of stars not predicted by our Galaxy model. Comparing the colours of the identified infrared counterparts with the model predictions, we observe that this excess is produced by yellow (G+K) stars. The study of the X-ray spectrum of each source reveals a main population of stars with the coronal temperature stratification typical of intermediate-age stars. As no assumptions have been made for the selection of the sample, our results must be representative of the entire solar neighbourhood. Some stars show infrared excess due to circumstellar absorption, which is indicative of youth. The XMM-Newton Bright Serendipitous Survey was conceived by the XMM-Newton Survey Science Center (SSC), a consortium of 10 institutions appointed by ESA to help the SOC in distinct technical aspects, including the exploitation of the XMM-Newton serendipitous detections (see http://xmmssc-www.star.le.ac.uk/). Tables 2 and 3 are only available in electronic form at http://www.aanda.org
Astronomy and Astrophysics, Volume 463, Issue 1, February III 2007, pp.165-174Feb 2007 - The onset of X-ray emission in young stellar objects. A Chandra observation of the Serpens star-forming region
Abstract
Aims: To study the properties of X-ray emission from young stellar objects (YSOs) through their evolution from Class I to Class III and determine whether Class 0 protostars emit X-rays. Methods: A deep Chandra X-ray observation of the Serpens star-forming region was obtained. The Serpens Cloud Core is ideally suited for this type of investigation, being populated by a dense and extremely young cluster whose members are found in all evolutionary stages, including six well-studied Class 0 sources. Results: None of the six Class 0 protostars is detected in our observations, excluding the presence of sources with typical X-ray luminosities ⪆ 0.4 × 1030 erg s-1 (for column densities of the order of 4 × 1023 cm-2, or AV ˜ 200). A total of 85 X-ray sources are detected and the light curves and spectra of 35 YSOs are derived. There is a clear trend of decreasing absorbing column densities as one moves from Class I to Class III sources, and some evidence of decreasing plasma temperatures, too. We observe a strong, long-duration, flare from a Class II low-mass star, for which we derive a flaring loop length of the order of 20 stellar radii. We interpret the flaring event as originating from a magnetic flux tube connecting the star to its circumstellar disk. The presence of such a disk is supported by the detection, in the spectrum of this star, of 6.4 keV Fe fluorescent emission. Section 6, Tables 5 and 6, and Fig. 13 are only available in electronic form at http://www.aanda.org
Astronomy and Astrophysics, Volume 463, Issue 1, February III 2007, pp.275-288Feb 2007 - The stellar population of the Rosat North Ecliptic Pole survey
Abstract
Context: X-ray surveys are a very efficient mean of detecting young stars and therefore allow us to study the young stellar population in the solar neighborhood and the local star formation history in the last billion of years. Aims: We want to study the young stellar population in the solar neighborhood, to constrain its spatial density and scale height as well as the recent local star formation history. Methods: We analyze the stellar content of the ROSAT North Ecliptic Pole survey, and compare the observations with the predictions derived from stellar galactic model. Since the ROSAT NEP survey is sensitive at intermediate fluxes is able to sample both the youngest stars and the intermediate age stars (younger than 109 years), linking the shallow and deep flux surveys already published in the literature. Results: We confirm the existence of an excess of yellow stars in our neighborhood previously seen in shallow survey, which is likely due to a young star population not accounted for in the model. However the excellent agreement between observations and predictions of dM stars casts some doubt on the real nature of this active population.
Astronomy and Astrophysics, Volume 461, Issue 3, January III 2007, pp.977-982Jan 2007 - Correlation between the spatial distribution of circumstellar disks and massive stars in the open cluster NGC 6611. Compiled catalog and cluster parameters
Abstract
Context: The observation of young stars with circumstellar disks suggests that the disks are dissipated, starting from the inner region, by the radiation of the central star and eventually by the formation of rocky planetesimals, over a time scale of several million years. It was also shown that strong UV radiation emitted by nearby massive stars can heat a circumstellar disk up to some thousand degrees, inducing the photoevaporation of the gas. This process strongly reduces the dissipation time scale. Aims: We study whether there exists a correlation between the spatial distribution of stars with circumstellar disks and the position of massive stars with spectral class earlier than B5, in the open cluster NGC 6611. Methods: We created a multiband catalog of the cluster, down to V˜ 23^m, using optical data from a WFI observation at 2.2 m of ESO in the BVI bands, the 2MASS public point source catalog and an archival X-ray observation made with CHANDRA/ACIS. We selected the stars with infrared excess (due to the emission of a circumstellar disk) using suitable color indices independent of extinction, and studied their spatial distribution. Results: The spatial distribution of the stars with K band excess (due to the presence of a circumstellar disk) is anti correlated with that of the massive stars: the disks are more frequent at large distances from these stars. We argue that this is in agreement with the hypothesis that the circumstellar disks are heated by the UV radiation from the massive stars and photoevaporated. Based on observations made with the European Observatory telescopes obtained from the ESO/ST-ECF Science Archive Facility. Table 3 and Appendix are only available in electronic form at http://www.aanda.org
Astronomy and Astrophysics, Volume 462, Issue 1, January IV 2007, pp.245-255Jan 2007
2006
- The young star cluster NGC 2362: low-mass population and initial mass function from a Chandra X-ray observation
Abstract
Context: . We study the stellar population of the very young cluster NGC 2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only 5 Myr old, has already cleared most of its inter- and circumstellar dust, and with its small and uniform reddening offers a unique opportunity of studying its pre-main-sequence stellar population with minimal disturbance from a dense interstellar medium. Aims: .Our main purposes are to select cluster members down to low masses and to study their properties as a population (spatial properties, initial mass function, and coronal properties). Methods: .We compare existing deep optical photometry and Hα data with new X-ray data. We use combined optical and X-ray criteria to select cluster members. Results: .We detect 387 X-ray sources down to log LX = 29.0 (erg/s), and identify most of them (308) with star-like objects. The majority (88%) of optically identified X-ray sources are found to be very good candidate low-mass pre-main-sequence stars, with minimal field-object contamination. This increases the known cluster census by a substantial amount at low masses, with respect to previous optical/IR studies. The fraction of stars with active accretion is found to be in the range 5-9%. We find a significantly wider spatial distribution for low-mass stars than for massive stars (mass segregation). We find only a small spread around the low-mass cluster sequence in the HR diagram, indicating that star formation lasted only about 1-2 Myr. We have derived the cluster initial mass function, which appears to flatten (on the low-mass side) at higher masses with respect to other very young clusters. The quiescent X-ray emission of low-mass cluster stars is found to be rather strictly correlated with the stellar bolometric luminosity: the small spread in this correlation puts an upper bound on the amplitude of X-ray variability on time scales longer than one day (e.g., activity cycles) in such young coronal sources. We find significant X-ray spectral differences between low-mass stars brighter and fainter than log LX ˜ 30.3 (erg/s), respectively, with X-ray brighter stars showing hotter components (kT ˜ 2 keV), absent in fainter stars.
Astronomy and Astrophysics, Volume 460, Issue 1, December II 2006, pp.133-144Dec 2006 - XMM-Newton survey of two upper Scorpius regions
Abstract
Aims.We studied X-ray emission from young stars by analyzing the deep XMM-Newton observations of two regions of the Upper Scorpius association with an age of 5 Myr. Methods: .Based on near infrared and optical photometry we identified 22 Upper Scorpius photometric members among the 224 detected X-ray sources. We derived coronal properties of Upper Scorpius stars by performing X-ray spectral and timing analyses. The study of four strong and isolated stellar flares allowed us to derive the length of the flaring loops. Results: .Among the 22 Upper Scorpius stars, 13 were identified as Upper Scorpius photometric members for the first time. The sample includes 7 weak-line T Tauri stars and 1 classical T Tauri star, while the nature of the remaining sources is unknown. Except for the intermediate mass star HD 142578, all the detected USco sources are low-mass stars of spectral types ranging from G to late M. The X-ray emission spectrum of the most intense Upper Scorpius sources indicates metal depleted plasma with temperature of ~10 MK, resembling the typical coronal emission of active main sequence stars. At least 59% of the detected members of the association have variable X-ray emission, and the flaring coronal structures appear shorter than or comparable to the stellar radii already at the Upper Scorpius age. We also found indications of increasing plasma metallicity (up to a factor 20) during strong flares. We identified a new galaxy cluster among the 224 X-ray source detected: the X-ray spectrum of its intra cluster medium indicates a redshift of ~0.41 ± 0.02.
Astronomy and Astrophysics, Volume 459, Issue 1, November III 2006, pp.199-213Nov 2006 - The rich young cluster NGC 6530: a combined X-ray-optical-infrared study
Abstract
We present a combined X-ray, optical, and IR (2MASS) study of NGC 6530, complementing our previous studies of this cluster. We consider different indicators of IR excesses, which can be taken as an indicator of circumstellar disks and therefore of pre-main-sequence status. We used reddening-free indices to ensure that our results are unaffected by highly irregular, differential reddening. More than the study of the JHK bands alone (33 IR-excess stars found), we found it useful to compare various optical and IR colors, finding overall 333 stars with optical-IR excess. In the field of the previously studied Chandra ACIS-I observation, we found 196 stars with optical-IR excess, of which 120 have not been previously found in X-rays. The total number of estimated cluster members thus becomes ≥1100. The estimated disk frequency in the ACIS field is ~20%. By considering different optical-IR excess indices, we find only partially overlapping subsamples, corresponding to different characteristics of their spectral energy distributions (longer- or shorter-wavelength IR excesses, or blue-violet excess). In a region displaced towards northwest with respect to the known cluster center, we find an unexpected concentration of stars with optical-IR excesses, most of which are not detected in X-rays. The spectral energy distribution of some of these objects shows more extreme excesses with respect to most ordinary T Tauri stars, which suggests circumstellar disks with very small inner holes and high accretion rates; some objects are best interpreted as being surrounded by reflection nebulosities, as found in some optically detected Class I objects in nearer star-forming regions. These reflection-nebulae candidates have the lowest X-ray detection rate among all subsamples considered here. Optical-IR excess stars in the north of NGC 6530 are nearly co-spatial with a sub-population of cluster stars older than the central cluster stars. This leads to the conclusion that in these northern regions of NGC 6530, far from massive cluster stars, star formation (and disk evolution) has proceeded rather undisturbed over longer time periods than near the cluster center, where most massive stars are found, and most stars lack substantial disks and strong accretion.
Astronomy and Astrophysics, Volume 459, Issue 2, November IV 2006, pp.477-488Nov 2006 - ACIS-I observations of NGC 2264. Membership and X-ray properties of PMS stars
Abstract
Aims.This paper's goal is to improve the member census of the NGC 2264 star-forming region and study the origin of X-ray activity in young PMS stars. Methods: .We analyze a deep, 100 ks long, Chandra ACIS observation covering a 17'×17' field in NGC 2264. The preferential detection in X-rays of low-mass PMS stars gives strong indications of their membership. We study X-ray activity as a function of stellar and circumstellar characteristics by correlating the X-ray luminosities, temperatures, and absorptions with optical and near-infrared data from the literature. Results: .We detect 420 X-ray point sources. Optical and NIR counterparts are found in the literature for 85% of the sources. We argue that more than 90% of these counterparts are NGC 2264 members, thereby significantly increasing the known low-mass cluster population by about 100 objects. Among the sources without counterpart, about 50% are probably associated with members, several of which we expect to be previously unknown protostellar objects. With regard to activity we confirm several previous findings: X-ray luminosity is related to stellar mass, although with a large scatter; L_X/L_bol is close to, but almost invariably below, the saturation level, 10-3, especially when considering the quiescent X-ray emission. A comparison between CTTS and WTTS shows several differences: CTTS have, at any given mass, activity levels that are both lower and more scattered than WTTS; emission from CTTS may also be more time variable and is on average slightly harder than for WTTS. However, we find evidence in some CTTS of extremely cool, ˜0.1-0.2 keV, plasma which we speculate is heated by accretion shocks. Conclusions: .Activity in low-mass PMS stars, while generally similar to that of saturated MS stars, may be significantly affected by mass accretion in several ways: accretion is probably responsible for very soft X-ray emission directly produced in the accretion shock; it may reduce the average energy output of solar-like coronae, at the same time making them hotter and more dynamic. We briefly speculate on a physical scenario that can explain these observations.
Astronomy and Astrophysics, Volume 455, Issue 3, September I 2006, pp.903-921Sep 2006 - X-ray and optical bursts and flares in YSOs: results from a 5-day XMM-Newton monitoring campaign of L1551
Abstract
We present the results of a five-day monitoring campaign with XMM-Newton of six X-ray bright young stellar objects (YSOs) in the star-forming complex L1551 in Taurus. All stars present significant variability on the five-day time scale. Modulation of the light curve on time scales comparable with the star's rotational period appeared to be present in the case of one weak-lined T Tauri star. Significant spectral variations between the 2000 and the 2004 observations were detected in the (unresolved) classical T Tauri binary system XZ Tau: a hot plasma component which was present in the X-ray spectrum in 2000 had significantly weakened in 2004. As XZ Tau N was undergoing a strong optical outburst in 2000, which had terminated since then, we speculate on the possible relationship between episodic, burst accretion, and X-ray heating. The transition object HL Tau underwent a strong flare with a complex temperature evolution, which is indicative of an event confined within a very large magnetic structure (few stellar radii), similar to the ones found in YSOs in the Orion Nebula Cluster.
Astronomy and Astrophysics, Volume 453, Issue 1, July I 2006, pp.241-252Jul 2006 - Chandra X-Ray Observations of Young Clusters. III. NGC 2264 and the Orion Flanking Fields
Abstract
Chandra observations of solar-like pre-main-sequence (PMS) stars in the Orion flanking fields (FFs; age ~1 Myr) and NGC 2264 (~3 Myr) are compared with the results of the Chandra Orion Ultradeep Project survey of similar objects in the Orion Nebula cluster (ONC; ~0.5 Myr). The correlations between logLX and mass found for PMS stars on convective tracks in these clusters are consistent with the relationships found for the ONC, indicating little change in the median values of either logLX or logLX/Lbol during the first ~3-5 Myr of evolution down convective tracks. The fraction of stars with extreme values of LX, more than 10 times higher than the average for a given Lbol, or with logLX/Lbol greater than the canonical saturation value of -2.9, is, however, larger by a factor of 2 in the younger ONC when compared with the Orion FFs and NGC 2264. PMS stars in NGC 2264 on radiative tracks have LX/Lbol values that are systematically lower by a factor of ~10 than those found for stars of similar mass on convective tracks. The dramatic decrease in flux from convective to radiative phases of PMS evolution is likely related to major structural changes that influence the efficiency of magnetic field generation and thus the level of magnetic activity. As in the ONC, we find that stars with measured periods have, on average, higher X-ray luminosities. However, there is a wide range in LX and LX/Lbol for both periodic and nonperiodic stars of similar mass. Among stars with measured periods, the level of X-ray emission does not correlate with the rotation rate. For this data set, we find no statistically significant correlation between X-ray flux and (1) the presence or absence of circumstellar accretion disks or (2) disk accretion rates as assessed from ultraviolet excesses.
The Astronomical Journal, Volume 131, Issue 6, pp. 2934-2948.Jun 2006 - Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton
Abstract
Aims.We report a deep X-ray survey of the young (~140 Myr), rich open cluster NGC 2516 obtained with the EPIC camera on board the XMM-Newton satellite. Methods: .By combining data from six observations, a high sensitivity, greater than a factor of 5 with respect to recent Chandra observations, has been achieved. Kaplan-Meier estimators of the cumulative X-ray luminosity distribution are built, statistically corrected for non members contaminants and compared to those of the nearly coeval Pleiades. The EPIC spectra of the X-ray brightest stars are fitted using optically thin model plasma with one or two thermal components. Results: .We detected 431 X-ray sources and 234 of them have as optical counterparts cluster stars spanning the entire NGC 2516 Main Sequence. On the basis of X-ray emission and optical photometry, we indicate 20 new candidate members of the cluster; at the same time we find 49 X-ray sources without known optical or infrared counterpart. The X-ray luminosities of cluster stars span the range log LX (erg s-1) = 28.4-30.8. The representative temperatures span the 0.3-0.6 keV (3.5-8 MK) range for the cool component and 1.0-2.0 keV (12-23 MK) for the hot one; similar values are found in other young open clusters like the Pleiades, IC 2391, and Blanco 1. While no significant differences are found in X-ray spectra, NGC 2516 solar type stars are definitely less luminous in X-rays than the nearly coeval Pleiades. The comparison with a previous ROSAT survey reveals the lack of variability amplitudes larger than a factor of 2 in solar type stars in a ˜ 11 yr time scale of the cluster and thus activity cycles like in the Sun are probably absent or have a different period and amplitude in young stars.
Astronomy and Astrophysics, Volume 450, Issue 3, May II 2006, pp.993-1004May 2006 - The discovery of an expanding X-ray source in the HH 154 protostellar jet
Abstract
Context: .Protostellar jets are a new class of X-ray sources which has been discovered with both XMM-Newton and Chandra. The mechanism responsible for the X-ray emission is still not clear. Self-shocking in jets, shocks where the jet hits the surrounding medium, reflected or scattered stellar X-ray emission have all been invoked as possible explanations. Aims: .One key diagnostic discriminating among physical emission mechanisms is the motion of the X-ray source: hydrodynamical numerical models of continuous protostellar jets plowing through a uniform medium show an X-ray emitting shock front moving at several hundreds km s-1. In the nearest X-ray emitting protostellar jet, HH 154, this is detectable, with the spatial resolution of the Chandra X-ray observatory, over a few years baseline, allowing a robust discrimination among different mechanisms. Methods: .We have performed, in October 2005, a deep Chandra X-ray observation of HH 154. Comparison with the previous (2001) Chandra observation allows to detect proper motion down to the level predicted by models of X-ray emitting shocks in the jet. Results: .The 2005 Chandra observation of HH 154 shows unexpected morphological changes of the X-ray emission in comparison with the 2001 data. Two components are present: a stronger, point-like component with no detectable motion and a weaker component which has expanded in size by approximately 300 AU over the 4 years time base of the two observations. This expansion corresponds to approximately 500 km s-1, very close to the velocity of the X-ray emitting shock in the simple theoretical models. Conclusions: .The 2005 data show a more complex system than initially thought (and modeled), with multiple components with different properties. The observed morphology is possibly indicating a pulsed jet propagating through a non-homogeneous medium, likely with medium density decreasing with distance from the driving source. Detailed theoretical modeling and deeper X-ray observations will be needed to understand the physics of this fascinating class of sources.
Astronomy and Astrophysics, Volume 450, Issue 1, April IV 2006, pp.L17-L20Apr 2006 - The discovery of an expanding X-ray source in the HH 154 protostellar jet
Abstract
Context: .Protostellar jets are a new class of X-ray sources which has been discovered with both XMM-Newton and Chandra. The mechanism responsible for the X-ray emission is still not clear. Self-shocking in jets, shocks where the jet hits the surrounding medium, reflected or scattered stellar X-ray emission have all been invoked as possible explanations. Aims: .One key diagnostic discriminating among physical emission mechanisms is the motion of the X-ray source: hydrodynamical numerical models of continuous protostellar jets plowing through a uniform medium show an X-ray emitting shock front moving at several hundreds km s-1. In the nearest X-ray emitting protostellar jet, HH 154, this is detectable, with the spatial resolution of the Chandra X-ray observatory, over a few years baseline, allowing a robust discrimination among different mechanisms. Methods: .We have performed, in October 2005, a deep Chandra X-ray observation of HH 154. Comparison with the previous (2001) Chandra observation allows to detect proper motion down to the level predicted by models of X-ray emitting shocks in the jet. Results: .The 2005 Chandra observation of HH 154 shows unexpected morphological changes of the X-ray emission in comparison with the 2001 data. Two components are present: a stronger, point-like component with no detectable motion and a weaker component which has expanded in size by approximately 300 AU over the 4 years time base of the two observations. This expansion corresponds to approximately 500 km s-1, very close to the velocity of the X-ray emitting shock in the simple theoretical models. Conclusions: .The 2005 data show a more complex system than initially thought (and modeled), with multiple components with different properties. The observed morphology is possibly indicating a pulsed jet propagating through a non-homogeneous medium, likely with medium density decreasing with distance from the driving source. Detailed theoretical modeling and deeper X-ray observations will be needed to understand the physics of this fascinating class of sources.
Astronomy and Astrophysics, Volume 450, Issue 1, April IV 2006, pp.L17-L20Apr 2006 - Chandra X-ray observations of the stellar group near the Herbig Be star MWC 297. A revision of the X-ray properties of MWC 297
Abstract
We present a Chandra ACIS-I X-ray observation of the region near the Herbig early-Be star MWC 297, where we detect a tight group of point X-ray sources. These are probably physically associated to MWC 297, because of their obvious clustering with respect to the more scattered field-source population. These data are compared to earlier ASCA data with much poorer spatial resolution, from which the detection of strong quiescent and flaring emission from MWC 297 itself was claimed. We argue that this star, contributing only 5% to the total X-ray emission of the group, was probably not the dominant contributor to the observed ASCA emission, while the X-ray brightest star in the group is a much better candidate. This is also supported by the spectral analysis of the Chandra data, with reference to the ASCA spectra. We conclude that none of the X-ray data available for MWC 297 justify the earlier claim of strong magnetic activity in this star. The X-ray emission of MWC 297 during the Chandra observation is even weaker than that found in other Herbig stars with the same spectral type, even accounting for its large line-of-sight absorption.
Astronomy and Astrophysics, Volume 447, Issue 3, March I 2006, pp.1041-1048Mar 2006
2005
- Chandra Orion Ultradeep Project: Observations and Source Lists
Abstract
We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep 2003 January Chandra X-Ray Observatory (Chandra) observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive data set on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.
The Astrophysical Journal Supplement Series, Volume 160, Issue 2, pp. 319-352.Oct 2005 - Rotational Modulation of X-Ray Emission in Orion Nebula Young Stars
Abstract
We investigate the spatial distribution of X-ray-emitting plasma in a sample of young Orion Nebula Cluster stars by modulation of their X-ray light curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP), is made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17'×17' field of view. We here focus on a subsample of 233 X-ray-bright stars with known rotational periods. We search for X-ray modulation using the Lomb Normalized Periodogram method. X-ray modulation related to the rotation period is detected in at least 23 stars with periods between 2 and 12 days and relative amplitudes ranging from 20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar rotation period, while in 7 cases it is about half that value, possibly due to the presence of X-ray-emitting structures at opposite stellar longitudes. These results constitute the largest sample of low-mass stars in which X-ray rotational modulation has been observed. The detection of rotational modulation indicates that the X-ray-emitting regions are distributed inhomogeneneously in longitude and do not extend to distances significantly larger than the stellar radius. Modulation is observed in stars with saturated activity levels (LX/Lbol~10-3) showing that saturation is not due to the filling of the stellar surface with X-ray-emitting regions. Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138. Universe Division, Science Mission Directorate, NASA Headquarters.
The Astrophysical Journal Supplement Series, Volume 160, Issue 2, pp. 450-468.Oct 2005 - Bright X-Ray Flares in Orion Young Stars from COUP: Evidence for Star-Disk Magnetic Fields?
Abstract
We have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 day observation of the Orion Nebula Cluster (ONC), concentrating on the events with the highest statistics (in terms of photon flux and event duration). Analysis of the flare decay allows to determine the physical parameters of the flaring structure, particularly its size and (using the peak temperature and emission measure of the event) the peak density, pressure, and minimum confining magnetic field. A total of 32 events, representing the most powerful ~=1% of COUP flares, have sufficient statistics and are sufficiently well resolved to grant a detailed analysis. A broad range of decay times are present in the sample of flares, with τlc (the 1/e decay time) ranging from 10 to 400 ks. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures that are found, from the analysis of the flare's decay, to confine the plasma are in a number of cases very long, with semilengths up to ~=1012 cm, implying the presence of magnetic fields of hundreds of G (necessary to confine the hot flaring plasma) extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures (length L>>R*) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L<=R*. As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures that confine the flaring plasma are actually the same type of structures that channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk.
The Astrophysical Journal Supplement Series, Volume 160, Issue 2, pp. 469-502.Oct 2005 - Chandra Orion Ultradeep Project Census of X-Ray Stars in the BN-KL and OMC-1S Regions
Abstract
We present a study of the X-ray sources detected in the vicinity of two density peaks in the Orion Molecular Cloud 1 (OMC-1) behind the Orion Nebula Cluster (ONC), as seen in the exceptionally deep (~10 days) exposure of the Chandra Orion Ultradeep Project (COUP). We focus on a 40''×50'' region around the Becklin-Neugebauer object and Kleinmann-Low nebula (collectively BN-KL) and a 60''×75'' region around OMC-1S, a secondary star-forming peak some 90" south of BN-KL. A total of 43 and 60 X-ray sources were detected in BN-KL and OMC-1S, respectively, of which one-half and one-third, respectively, were found to be foreground members of the ONC, while the remaining sources are identified as obscured X-ray sources with column densities 22<~log(NH/cm-2)<~24. All but 5 and 18 of these sources have near-infrared stellar counterparts, and 22 of these appear to be young stellar objects (YSOs). X-ray sources are found close to four luminous mid-IR sources, namely, BN, IRc3-i2, IRc2-C, and source n; their X-ray variability and spectral properties are typical of coronal activity in low-mass stars rather than wind emission from massive stars, suggesting that the X-ray emission may be arising from companions. The X-ray light curve of the X-ray source close to BN shows a periodicity of ~8.3 days, and from an X-ray image deconvolution of the region around BN, we conclude that either BN itself or a low-mass companion with a projected separation of ~=200 AU was detected. On the other hand, no emission is seen from the bright radio source I, held by some to be the main source of luminosity in BN-KL. In OMC-1S, Chandra unveils a new subcluster of seven YSOs without infrared counterparts. We compare the hard-band X-ray luminosity functions of obscured X-ray sources in BN-KL and OMC-1S with unobscured X-ray sources in the ONC, and we estimate that the true population of obscured sources in BN-KL and OMC-1S is ~=46 and 57 stars, with 90% confidence intervals of 34-71 and 42-82 stars, respectively.
The Astrophysical Journal Supplement Series, Volume 160, Issue 2, pp. 530-556.Oct 2005 - X-Ray Emission from Early-Type Stars in the Orion Nebula Cluster
Abstract
The X-ray properties of twenty ~1 Myr old O, B, and A stars of the Orion Trapezium are examined with data from the Chandra Orion Ultradeep Project (COUP). On the basis of simple theories for X-ray emission, we define two classes separated at spectral type B4: hotter stars have strong winds that may give rise to X-ray emission in small- or large-scale wind shocks, and cooler stars that should be X-ray dark due to their weaker winds and absence of outer convection zones where dynamos can generate magnetic fields. Emission by late-type magnetically active companions may be present in either class. Sixteen of the 20 stars are detected with a wide range of X-ray luminosities, logLX (ergs s-1)~29-33, and X-ray efficiencies, log(LX/Lbol)~-4 to -8. Only two stars, θ1 Ori D (B0.5) and NU Ori (B1), show exclusively the constant soft-spectrum emission at log(LX/Lbol)~-7 expected from the standard model involving many small shocks in an unmagnetized radiatively accelerated wind. Most of the other massive O7-B3 stars exhibit some combination of soft-spectrum wind emission, hard-spectrum flaring, and/or rotational modulation indicating large-scale inhomogeneity. Magnetic confinement of winds with large-scale shocks can be invoked to explain these phenomena. This is supported in some cases by nonthermal radio emission and/or chemical peculiarities, or direct detection of the magnetic field (θ1 Ori C). Most of the stars in the weak-wind class exhibit X-ray flares and logLX<31 ergs s-1, consistent with magnetic activity from known or unseen low-mass companions. In most cases, the X-ray spectra can be interpreted in terms of a two-temperature plasma model with a soft component of 3-10 MK and a hard component up to 40 MK. All nondetections belong to the weak-wind class. A group of stars exhibit hybrid properties-flarelike behavior superimposed on a constant component with logLX~32 ergs s-1-which suggest both magnetic activity and wind emission.
The Astrophysical Journal Supplement Series, Volume 160, Issue 2, pp. 557-581.Oct 2005 - X-Ray Emission from Young Brown Dwarfs in the Orion Nebula Cluster
Abstract
We use the sensitive X-ray data from the Chandra Orion Ultradeep Project (COUP) to study the X-ray properties of 34 spectroscopically identified brown dwarfs with near-infrared spectral types between M6 and M9 in the core of the Orion Nebula Cluster. Nine of the 34 objects are clearly detected as X-ray sources. The apparently low detection rate is in many cases related to the substantial extinction of these brown dwarfs; considering only the brown dwarfs with AV<=5 mag, nearly half of the objects (7 out of 16) are detected in X-rays. Our 10 day long X-ray light curves of these objects exhibit strong variability, including numerous flares. While one of the objects was only detected during a short flare, a statistical analysis of the light curves provides evidence for continuous (``quiescent'') emission in addition to flares for all other objects. Of these, the ~M9 brown dwarf COUP 1255 (=HC 212) is one of the coolest known objects with a clear detection of quiescent X-ray emission. The X-ray properties (spectra, fractional X-ray luminosities, flare rates) of these young brown dwarfs are similar to those of the low-mass stars in the Orion Nebula Cluster, and thus there is no evidence for changes in the magnetic activity around the stellar/substellar boundary, which lies at ~M6 for Orion Nebula Cluster sources. Since the X-ray properties of the young brown dwarfs are also similar to those of M6-M9 field stars, the key to the magnetic activity in very cool objects seems to be the effective temperature, which determines the degree of ionization in the atmosphere.
The Astrophysical Journal Supplement Series, Volume 160, Issue 2, pp. 582-593.Oct 2005 - A survey for Fe 6.4 keV emission in young stellar objects in ρ Oph: The strong fluorescence from Elias 29
Abstract
We report the results of a search for 6.4 keV Fe fluorescent emission in Young Stellar Objects (YSOs) with measured accretion luminosities in the ρ Oph cloud, using the existing Chandra and XMM-Newton observations of the region. A total of nine such YSOs have X-ray data with sufficiently high S/N for the 6.4 keV line to be potentially detected if present. A positive detection of the Fe 6.4 keV line is reported for one object, Elias 29, in both the XMM-Newton and the Chandra data. The 6.4 keV line is detected in Elias 29 both during quiescent and flaring emission, unlikely all previously reported detections of 6.4 keV Fe fluorescence in YSOs which were made during intense flaring. The observed equivalent width of the fluorescent line is large, Wα ≃ 160 eV, ruling out fluorescence from diffuse circumstellar material. It is also larger than expected for simple reflection from a solar-composition photosphere or circumstellar disk, but it is compatible with being due to fluorescence from a centrally illuminated circumstellar disk. The X-ray spectrum of Elias 29 is also peculiar in terms of its high (ionized) Fe abundance, as evident from the very intense Fe xxv 6.7 keV line emission; we speculate on the possible mechanism leading to the observed high abundance.
Astronomy and Astrophysics, Volume 433, Issue 3, April III 2005, pp.1047-1054Apr 2005 - The star formation region NGC 6530: Distance, ages and initial mass function
Abstract
We present astrometry and BVI photometry, down to V≃22, of the very young open cluster NGC 6530, obtained from observations taken with the Wide Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the V vs. B-V and the V vs. V-I color-magnitude diagrams (CMD) show that the upper main sequence is dominated by very bright cluster stars, while, because of the high obscuration of the giant molecular cloud surrounding the cluster, the blue envelopes of the diagrams at V≳14 are limited to the main sequence stars at the distance of NGC 6530. This particular structure of the NGC 6530 CMD allows us to conclude that its distance is about d ≃ 1250 pc, significantly lower than the previous determination of d=1800 pc. We have positionally matched our optical catalog with the list of X-ray sources found in a Chandra-ACIS observation, finding a total of 828 common stars, 90% of which are pre-main sequence stars in NGC 6530. Using evolutionary tracks of \citet{sies00}, mass and age values are inferred for these stars. The median age of the cluster is about 2.3 Myr; in the mass range (0.6-4.0) M⊙, the Initial Mass Function (IMF) shows a power law index x=1.22±0.17, consistent with both the Salpeter index (1.35), and with the index derived for other young clusters; towards smaller masses the IMF shows a peak and then it starts to decrease. Based on observations made with the European Southern Observatory telescopes obtained from the ESO/ST-ECF Science Archive Facility. Full Tables 4 and 5 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/941
Astronomy and Astrophysics, v.430, p.941-957 (2005)Feb 2005 - XMM-Newton observation of the young open cluster Blanco 1. II. X-ray time variability and flares
Abstract
We study the X-ray variability of the young open cluster Blanco 1 observed with the EPIC camera on board the XMM-Newton X-ray observatory. The time coverage of EPIC observations has allowed us to address short time scale (hours) variability, while the comparison with previous ROSAT observations has allowed us to investigate the variability on time scale of six years. On the time scale of hours, dM stars of the cluster are more variable than solar-mass stars. The main features of X-ray light curves in dM stars appear to be essentially flare-like events with a typical duration of the order of a few ks, while dF-dG stars show smooth variations. Two intense flares were observed in the ZS76 cluster member. From the analysis of the light curves and of the time resolved X-ray spectroscopy we infer that they occurred in arcades, made of several similar coronal loops (˜1010 cm long), ignited at ˜15 ks time delay. Minor heat pulses may occur in the first flare. The similarity of the two flares and of the flaring inferred structures may suggest that the flares are physically connected to each other.} Amplitude variations on the time scale of 6 years are smaller by a factor 2, whereas a factor of 10 is observed on the Sun in the same band. This implies a smaller amplitude of any analog of the 11-yr cycle in these young active coronae. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. flares
Astronomy and Astrophysics, v.430, p.155-164 (2005)Jan 2005 - X-ray spectral and timing characteristics of the stars in the young open cluster IC 2391
Abstract
We present X-ray spectral and timing analysis of members of the young open cluster IC 2391 observed with the XMM-Newton observatory. We detected 99 X-ray sources by analysing the summed data obtained from MOS1, MOS2 and pn detectors of the EPIC camera; 24 of them are members, or probable members, of the cluster. Stars of all spectral types have been detected, from the early-types to the late-M dwarfs. Despite the capability of the instrument to recognize up to 3 thermal components, the X-ray spectra of the G, K and M members of the cluster are well described with two thermal components (at kT1 ˜ 0.3-0.5 keV and kT2 ˜ 1.0-1.2 keV respectively) while the X-ray spectra of F members require only a softer 1-T model. The Kolmogorov-Smirnov test applied to the X-ray photon time series shows that approximately 46% of the members of IC 2391 are variable with a confidence level >99%. The comparison of our data with those obtained with ROSAT/PSPC, nine years earlier, and ROSAT/HRI, seven years earlier, shows that there is no evidence of significant variability on these time scales, suggesting that long-term variations due to activity cycles similar to that on the Sun are not common, if present at all, among these young stars.
Astronomy and Astrophysics, v.430, p.287-301 (2005)Jan 2005
2004
- X-Ray Spectroscopy of the Unsteady Quiescent Corona of AD Leonis with Chandra
Abstract
We present the analysis and interpretation of an observation of the flare star AD Leo (dM3e) with the Low Energy Transmission Grating of Chandra. The high-resolution X-ray spectrum-dominated by emission lines from O VII-VIII, Ne IX-X, and Fe XVII-Fe XIX-allowed us to infer the plasma emission measure distribution (EMD) versus temperature, as well as the abundances of individual elements in the corona of this magnetically active star, during a typical state characterized by significant variability but no evident flaring event. We have also measured plasma densities at various temperatures using spectroscopic diagnostics provided by He-like triplets and Fe XXI lines. We show that the present EMD is similar, in terms of overall shape and temperature of the peak, to those previously obtained from Extreme Ultraviolet Explorer (EUVE) spectra during quiescent and flaring states confirming the long-term stability of the corona of AD Leo. At variance with the case of other active stars, the EMD of AD Leo is characterized by a significantly shallower slope, compatible with that predicted by static models of isobaric loops with constant cross section and uniform heating. We discuss such coronal modeling to infer the average properties of the corona in terms of loop populations, including estimates of the surface filling factor derived by comparison between the model and the observed EMD. We also show that the EMD is compatible with the model of a corona continuously heated by flares, which predicts an EMD slope slightly steeper than observed, but that can be accommodated by observational uncertainties. The coronal composition is such that the element abundances, relative to solar values, tend to increase with the first ionization potential, with few exceptions. The line-to-continuum ratios suggest a nearly solar metallicity, a result difficult to reconcile with previous determinations based on global fitting of X-ray spectra.
The Astrophysical Journal, Volume 613, Issue 1, pp. 548-566.Sep 2004 - The Chandra Deep Field-North Survey. XVII. Evolution of Magnetic Activity in Old Late-Type Stars
Abstract
The extremely sensitive Chandra Deep Field-North (CDF-N) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main-sequence Galactic stars. Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spin-down of stellar rotation from a magnetized stellar wind. Thirteen X-ray sources are associated with late-type stars; 11 of these constitute a well-defined sample for statistical analysis. This sample consists of two G, two K0-K4, and seven M2-M5 stars with median V-band magnitude around 19 and median distance around 300 pc. X-ray luminosities are typically logLX~=27 ergs s-1 but are substantially higher in two cases. The combination of large-amplitude variations on timescales of hours and plasma temperatures around 5-30 MK indicates that the observed X-ray emission is dominated by magnetic reconnection flares rather than quiescent coronal emission. These X-ray properties are quantitatively similar to those seen in the active contemporary Sun. The CDF-N stellar sample is compared to simulations based on convolution of X-ray luminosity functions (XLFs) with the known spatial distribution of old-disk stars. The model indicates that the CDF-N stars are the most magnetically active old-disk stars. A substantial decline in X-ray luminosities over the 1Gyr<t<11Gyr age interval is required: 39 rather than 11 stars should have been detected if the XLF does not evolve over this time interval. This is a clear demonstration that the coronal and flaring components of stellar magnetic activity-and presumably the interior magnetic dynamos responsible for the reconnecting fields at the stellar surface-exhibit long-term decay over the age of the Galactic disk. The model that best fits the magnitudes, spectral types, and X-ray luminosities of the sample has LX~t-2 ergs s-1, which is faster than the t-1 decay rate predicted from widely accepted rotational spin-down rates and X-ray-activity relations. Based in part on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.
The Astrophysical Journal, Volume 611, Issue 2, pp. 1107-1120.Aug 2004 - High-Resolution X-Ray Spectroscopy of the Post-T Tauri Star PZ Telescopii
Abstract
We present an analysis of the Chandra High Energy Transmission Grating Spectrometer observation of the rapidly rotating (Prot=0.94days) post-T Tauri (~20 Myr old) star PZ Telescopii, in the Tucana association. Using two different methods, we have derived the coronal emission measure distribution EM(T) and chemical abundances. The EM(T) peaks at logT=6.9 and exhibits a significant emission measure at temperatures logT>7. The coronal abundances are generally ~0.5 times the solar photospheric values, which are presumed fairly representative of the composition of the underlying star. A minimum in abundance is seen at a first ionization potential (FIP) of 7-8 eV, with evidence for higher abundances at both lower and higher FIP, similar to patterns seen in other active stars. From an analysis of the He-like triplet of Mg XI, we have estimated electron densities of ~1012-1013 cm-3. All the coronal properties found for PZ Tel are much more similar to those of AB Dor, which is slightly older than PZ Tel, than to those of the younger T Tauri star TW Hya. These results support earlier conclusions that the soft X-ray emission of TW Hya is likely dominated by accretion activity rather than by a magnetically heated corona. Our results also suggest that the coronae of pre-main-sequence stars rapidly become similar to those of older active main-sequence stars soon after the accretion stage has ended.
The Astrophysical Journal, Volume 609, Issue 2, pp. 925-934.Jul 2004 - XMM-Newton observations of the young open cluster Blanco 1. I. X-ray spectroscopy and photometry
Abstract
We present an X-ray study of the high metallicity young open cluster Blanco 1 based on XMM-Newton data. X-ray spectroscopy of cluster members is presented for the first time as well as new X-ray distribution functions of late-type stars. We detected all known dF and dG stars in the EPIC field and 80% and 90% of dK and dM stars, respectively. The X-ray spectral analysis of the X-ray brightest cluster stars and X-ray color analysis of a larger sample show that a model with two temperatures (at about 0.3 and 1 keV) explains the quiescent activity phase spectra. We discuss also the nature of unidentified X-ray sources in the observed region and their X-ray spectral properties. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. Appendix B is only available in electronic form at http://www.edpsciences.org
Astronomy and Astrophysics, v.421, p.175-186 (2004)Jul 2004 - A Deep Chandra X-Ray Observation of the Rich Young Cluster NGC 6530. I. The X-Ray Source Catalog and the Cluster Population
Abstract
In a deep 60 ks Chandra ACIS X-ray observation of the very young cluster NGC 6530, we detect 884 X-ray point sources and argue that a very large fraction of them (90%-95%) must be pre-main-sequence (PMS) cluster members, mostly of low masses. This is a significant enlargement of the known NGC 6530 stellar population with respect to previous optical studies, including Hα surveys. We identify 220 X-ray sources with cataloged stars down to V=17, while most unidentified sources have fainter counterparts. Moreover, we find an infrared counterpart in the 2MASS catalog for 731 X-ray sources. The optically identified cluster X-ray sources are found in a band in the H-R diagram above the main sequence, in the locus of 0.5-1.5 Myr PMS stars, with masses down to 0.5-1.5 Msolar. We find evidence of an age gradient across the field from northwest to south, suggesting a sequence of star formation events qualitatively similar to that found in earlier studies of the same region, but differing in the details. A group of X-ray sources showing frequent flares may be associated with the youngest stars in the cluster, suggesting that X-ray flaring activity is especially intense in the youngest PMS phases of low-mass stars.
The Astrophysical Journal, Volume 608, Issue 2, pp. 781-796.Jun 2004 - Coronal Variability in the Young Cluster NGC 2516
Abstract
NGC 2516 has been observed by Chandra several times in order to correct the plate scale of the spacecraft's focal plane instruments. Because of this, Chandra has observed NGC 2516 with all four imaging arrangements available. In addition, NGC 2516 has been observed as part of the High Resolution Camera (HRC) guaranteed time program and is scheduled for return plate scale calibration visits. This makes it the best cluster to study for long-term variability. NGC 2516 is about 140 Myr old and less than 400 pc away. In our first paper, we discussed the detection of 150 X-ray sources (42% of which are identified as cluster members) in the calibration data taken during the orbital activation phase of the Chandra mission. In our second paper, we combined all the extant data sets and detected 284 sources, more than half of which are considered likely cluster members. In this our third paper, we further explore techniques of combining Advanced CCD Imaging Spectrometer (ACIS) and HRC Chandra data for timing analysis. We have been able to combine almost 70 ks of observation time, spread over five epochs, to study variability in this cluster on multiple timescales. We find that while stochastic variability rates are about the same for all objects in the sample, the timescale for detecting variability is shorter for late-type stars. Both stochastic and flare variability rates seen in NGC 2516 are similar to those seen in younger clusters IC 348, NGC 1333, and M42.
The Astrophysical Journal, Volume 606, Issue 1, pp. 466-482.May 2004 - Parameter properties and stellar population of the old open cluster NGC 3960
Abstract
We present a BV I photometric and astrometric catalogue of the open cluster NGC 3960, down to limiting magnitude V˜22, obtained from observations taken with the Wide Field Imager camera at the MPG/ESO 2.2 m Telescope at La Silla. The photometry of all the stars detected in our field of view has been used to estimate a map of the strong differential reddening affecting this area. Our results indicate that, within the region where the cluster dominates, the E(V-I) values range from 0.21 up to 0.78, with E(V-I)=0.36 (E(B-V)=0.29) at the nominal cluster centroid position; color excesses E(V-I) up to 1 mag have been measured in the external regions of the field of view where field stars dominate. The reddening-corrected color-magnitude diagram (CMD) allows us to conclude that the cluster has an age between 0.9 and 1.4 Gyr and a distance modulus of (V-M_V)_0=11.35. In order to minimize field star contamination, their number has been statistically subtracted based on the surface density map. The empirical cluster main sequence has been recovered in the V vs. V-I and in the J vs. J-K_S planes, using optical and infrared data, respectively. From these empirical cluster main sequences, two samples of candidate cluster members were derived in order to obtain the luminosity distributions as functions of the V and J magnitudes. The Luminosity Functions have been transformed into the corresponding Mass Functions; for M>1 M_⊙, the two distributions have been fitted with a power law of index αV=2.95±0.53 and αJ=2.81±0.84 in V and in J, respectively, while the Salpeter Mass Function in this notation has index α=2.35. Based on observations made with the European Southern Observatory telescopes obtained from the ESO/ST-ECF Science Archive Facility. Tables 5 and 6 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/417/945
Astronomy and Astrophysics, v.417, p.945-960 (2004)Apr 2004 - High-amplitude, long-term X-ray variability in the solar-type star HD 81809: The beginning of an X-ray activity cycle?
Abstract
We present the initial results from our XMM-Newton program aimed at searching for X-ray activity cycles in solar-type stars. HD 81809 is a G2-type star (somewhat more evolved than the Sun, and with a less massive companion) with a pronounced 8.2 yr chromospheric cycle, as evident from from the Mt. Wilson program data. We present here the results from the initial 2.5 years of XMM-Newton observations, showing that large amplitude (a factor of ≃10) modulation is present in the X-ray luminosity, with a clearly defined maximum in mid 2002 and a steady decrease since then. The maximum of the chromospheric cycle took place in 2001; if the observed X-ray variability is the initial part of an X-ray cycle, this could imply a phase shift between chromospheric and coronal activity, although the current descent into chromospheric cycle minimum is well reflected into the star's X-ray luminosity. The observations presented here provide clear evidence for the presence of large amplitude X-ray variability coherent with the activity cycle in the chromosphere in a star other than the Sun.
Astronomy and Astrophysics, v.418, p.L13-L16 (2004)Apr 2004 - Studying the evolution of the hot universe with the X-ray evolving universe spectroscopy mission – XEUS
Abstract
Europe is one of the major partners building the International Space Station (ISS) and European industry, together with ESA, is responsible for many station components including the Columbus Orbital Facility, the Automated Transport Vehicle, two connecting modules and the European Robotic Arm. Together with this impressive list of contributions there is a strong desire within the ESA Member States to benefit from this investment by utilizing the unique capabilities of the ISS to perform world-class science. XEUS is one of the astronomical applications being studied by ESA to utilize the capabilities of the ISS. XEUS will be a long-term X-ray observatory with an initial mirror area of 6 m 2 at 1 keV that will be expanded to 30 m 2 following a visit to the ISS. The 1 keV spatial resolution is expected to be 2-5″ half-energy-width. XEUS will consist of separate detector and mirror spacecraft (MSC) aligned by active control to provide a focal length of 50 m. A new detector spacecraft, complete with the next generation of instruments, will also be added after visiting the ISS. The limiting 0.1-2.5 keV sensitivity will then be 4 × 10 -18 erg cm -2 s -1, around 200 times better than XMM- Newton, allowing XEUS to study the properties of the hot baryons and dark matter at high redshift.
Advances in Space Research, Volume 34, Issue 12, p. 2623-2627.Jan 2004
2003
- X-ray variability of Pleiades late-type stars as observed with the ROSAT-PSPC
Abstract
We present a comprehensive analysis of X-ray variability of the late-type (dF7-dM) Pleiades stars, detected in all ROSAT-PSPC observations; X-ray variations on short (hours) and medium (months) time scales have been explored. We have grouped the stars in two samples: 89 observations of 42 distinct dF7-dK2 stars and 108 observations of 61 dK3-dM stars. The Kolmogorov-Smirnov test applied on all X-ray photon time series show that the percentage of cases of significant variability is quite similar on both samples, suggesting that the presence of variability does not depend on mass for the time scales and mass range explored. The comparison between the Time X-ray Amplitude Distribution functions (XAD) of the set of dF7-dK2 and of the dK3-dM show that, on short time scales, dK3-dM stars show larger variations than dF7-dK2. A subsample of eleven dF7-dK2 and eleven dK3-dM Pleiades stars allows the study of variability on longer time scales: we found that variability on medium - long time scales is relatively more common among dF7-dK2 stars than among dK3-dM ones. For both dF7-dK2 Pleiades stars and dF7-dK2 field stars, the variability on short time scales depends on Lx while this dependence has not been observed among dK3-dM stars. It may be that the variability among dK3-dM stars is dominated by flares that have a similar luminosity distribution for stars of different Lx, while flaring distribution in dF7-dK2 stars may depend on X-ray luminosity. The lowest mass stars show significant rapid variability (flares?) and no evidence of rotation modulation or cycles. On the contrary, dF7-dK2 Pleiades stars show both rapid variability and variations on longer time scales, likely associated with rotational modulation or cycles.
Astronomy and Astrophysics, v.406, p.629-638 (2003)Aug 2003 - X-ray rotational modulation of a supersaturated star in IC 2391
Abstract
We present evidence of X-ray rotational modulation on VXR45, a young fast rotator star, member of IC 2391. It is a dG9 spectral type star whose rotational period and X-ray luminosity make it a supersaturated star. Our X-ray observation, made with EPIC/PN on XMM-Newton, covers about two photometric rotational periods. The detection of X-ray rotational modulation implies the presence of structural inhomogeneities. Possible interpretations are presented and discussed.
Astronomy and Astrophysics, v.407, p.L63-L66 (2003)Aug 2003 - Luminosity and Mass Function of the Galactic open cluster NGC 2422
Abstract
We present UBVRI photometry of the open cluster NGC 2422 (age ~ 108 yr) down to a limiting magnitude V =~ 19. These data are used to derive the Luminosity and Mass Functions and to study the cluster spatial distribution. By considering the color-magnitude diagram data and adopting a representative cluster main sequence, we obtained a list of candidate cluster members based on a photometric criterion. Using a reference field region and an iterative procedure, a correction for contaminating field stars has been derived in order to obtain the Luminosity and the Mass Functions in the M=0.4-3.5 Msun range. By fitting the spatial distribution, we infer that a non-negligible number of cluster stars lies outside our investigated region. We have estimated a correction to the Mass Function of the cluster in order to take into account the ``missing'' cluster stars. The Present Day Mass Function of NGC 2422 can be represented by a power-law of index alpha = 3.07 +/-0.08 (rms) - the Salpeter Mass Function in this notation has index alpha = 2.35 - in the mass range 0.9 <= M/Msun<= 2.5 . The index alpha and the total mass of the cluster are very similar to those of the Pleiades. Based on observations made at the Cerro Tololo Inter-American Observatory, Chile.
Astronomy and Astrophysics, v.404, p.927-937 (2003)Jun 2003 - X-Ray Rotational Modulation in VXR45
Abstract
We present evidence of X-ray rotational modulation on VXR45, a young very fast rotator star member of IC2391 open cluster, observed with XMM-Newton observatory.
Information Bulletin on Variable Stars, No. 5427, #1.Jun 2003 - Chandra X-Ray Observations of the Young Open Cluster NGC 2516
Abstract
We present a comprehensive study of the Chandra X-ray observations of the young open cluster NGC 2516. We have analyzed eight individual Chandra observations. We have combined the data to achieve the greatest sensitivity, reaching down to logfX=-14.56 (ergs s-1 cm -2), or logLX=28.69 (ergs s-1) at the distance of NGC 2516. Out of 284 X-ray sources detected, 155 are identified with photometric cluster members, with very little ambiguity. We have studied the X-ray luminosity functions for the various spectral types, correcting for nonmember contamination those for later type stars. We find strong X-ray emission from a couple of B stars that cannot be due to companions. Among A stars, chemically peculiar stars show a detection fraction much larger than normal A stars. The luminosity functions decrease monotonically from F to M stars, and those for K stars are significantly lower than similar stars in the Pleiades, a cluster only slightly younger than NGC 2516. We discuss possible causes of this discrepancy, including differences in age and stellar rotation.
The Astrophysical Journal, Volume 588, Issue 2, pp. 1009-1024.May 2003 - An XMM-Newton-based X-ray survey of pre-main sequence stellar emission in the L1551 star-forming complex
Abstract
We present a study of the X-ray sources present in the nearby L1551 star forming region, based on a deep XMM-Newton observation complemented with Chandra data for the brightest sources. Most known pre-main sequence stars in the region have been detected, most of them with sufficient statistics to allow a detailed study of the temporal and spectral characteristics of their X-ray emission. Significant temporal (and spectral) variability on both short and long time scales is visible for most of the stars. In particular XZ Tau shows large-amplitude variations on time scales of several hours with large changes in the intervening absorption, suggestive of the X-ray emission being eclipsed by the accretion stream (and thus of the X-ray emission being partly or totally accretion-induced). The coronal metal abundance of the WTTS sources is clustered around Z =~ 0.2, while the CTTS sources span almost two orders of magnitudes in coronal Z, even though the photospheric abundance of all stars in the L1551 is likely to be very similar. Some individual elements (notably Ne) appear to be systematically enhanced with respect to Fe in the WTTS stars. The significant differences between the spectral and temporal characteristics of the CTTS and WTTS populations suggest that a different emission mechanism is (at least partly) responsible for the X-ray emission of the two types of stars.
Astronomy and Astrophysics, v.403, p.187-203 (2003)May 2003 - Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks
Abstract
We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), la 1 Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a ~ 1 Msun star. We consider five SFR of varying age (rho Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I, and eta Chamaeleontis) and two young clusters (the Pleiades and NGC 2516). Optical and X-ray data for these regions are retrieved both from archival observations and recent literature, and reanalyzed here in a consistent manner so as to minimize systematic differences in the results. We study trends of LX and LX/Lbol as a function of stellar mass and association age. For low mass stars (M <~ 1 Msun) we observe an increase in LX/Lbol in the first 3-4 Myr and a subsequent leveling off at the saturation level (LX/Lbol ~ -3). Slowly evolving very low mass stars then retain saturated levels down to the oldest ages here considered, while for higher mass stars activity begins to decline at some age after ~ 107 years. We find our data consistent with the following tentative picture: low mass PMS stars with no circumstellar accretion disk have saturated activity, consistently with the activity-Rossby number relation derived for MS stars. Accretion and/or the presence of disks somehow lowers the observed activity levels; disk dissipation and/or the decrease of mass accretion rate in the first few Myrs of PMS evolution is therefore responsible for the observed increase of LX/Lbol with time. Table 1 is only available in electronic form at http://www.edpsciences.org
Astronomy and Astrophysics, v.402, p.277-292 (2003)Apr 2003 - The X-ray Luminosity Distributions of the high-metallicity open cluster Blanco 1
Abstract
We present X-ray Luminosity Distributions (XLDs) of late-type members (dF, dG, dK, dM) of the Blanco 1 cluster, based on ROSAT-HRI data and new astrometric-photometric membership obtained from the GSC-II project. For the first time we present the XLD of dM stars of this cluster. The high metallicity of Blanco 1 allows us to investigate the role of chemical composition on the coronal emission of late-type stars. Comparison between X-ray Luminosity Distributions of Blanco 1 and Pleiades, NGC 2516 and alpha Per suggests a possible metallicity effect in dM stars.
Astronomy and Astrophysics, v.399, p.919-929 (2003)Mar 2003 - The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs
Abstract
We present the results of a new study on the relationship between coronal X-ray emission and stellar rotation in late-type main-sequence stars. We have selected a sample of 259 dwarfs in the B-V range 0.5-2.0, including 110 field stars and 149 members of the Pleiades, Hyades, alpha Persei, IC 2602 and IC 2391 open clusters. All the stars have been observed with ROSAT, and most of them have photometrically-measured rotation periods available. Our results confirm that two emission regimes exist, one in which the rotation period is a good predictor of the total X-ray luminosity, and the other in which a constant saturated X-ray to bolometric luminosity ratio is attained; we present a quantitative estimate of the critical rotation periods below which stars of different masses (or spectral types) enter the saturated regime. In this work we have also empirically derived a characteristic time scale, taue , which we have used to investigate the relationship between the X-ray emission level and an X-ray-based Rossby number Re = Prot/taue: we show that our empirical time scale taue resembles the theoretical convective turnover time for 0.4 <~ M/Msun <~ 1.2, but it also has the same functional dependence on B-V as Lbol-1/2 in the color range 0.5 <~ B-V <~ 1.5. Our results imply that - for non-saturated coronae - the Lx - Prot relation is equivalent to the Lx/Lbol vs. Re relation. Tables 1 and 2 are only available in electronic form at \ http://www.edpsciences.org
Astronomy and Astrophysics, v.397, p.147-157 (2003)Jan 2003 - Chandra X-Ray Observation of the Orion Nebula Cluster. I. Detection, Identification, and Determination of X-Ray Luminosities
Abstract
In this first of two companion papers on the Orion Nebula Cluster (ONC), we present our analysis of a 63 ks Chandra HRC-I observation that yielded 742 X-ray detections within the 30'×30' field of view. To facilitate our interpretation of the X-ray image, here we collect a multiwavelength catalog of nearly 2900 known objects in the region by combining 17 different catalogs from the recent literature. We define two reference groups: an infrared sample, containing all objects detected in the K band, and an optical sample comprising low-extinction, well-characterized ONC members. We show for both samples that field object contamination is generally low. Our X-ray sources are primarily low-mass ONC members. The detection rate for optical sample stars increases monotonically with stellar mass from zero at the brown dwarf limit to ~100% for the most massive stars but shows a pronounced dip between 2 and 10 Msolar. We determine LX and LX/Lbol for all stars in our optical sample and utilize this information in our companion paper to study correlations between X-ray activity and other stellar parameters.
The Astrophysical Journal, Volume 582, Issue 1, pp. 382-397.Jan 2003 - Chandra X-Ray Observation of the Orion Nebula Cluster. II. Relationship between X-Ray Activity Indicators and Stellar Parameters
Abstract
Using the results of our first paper on the Chandra HRC observation of the Orion Nebula cluster (ONC), we explore the relation between the coronal activity of its 1 Myr-old pre-main-sequence population and stellar parameters. We find that median X-ray luminosities of low-mass stars (M/Msolar<~3) increase with increasing mass and decreasing stellar age. Brown dwarfs (0.03<~M/Msolar<~0.08) follow the same trend with mass. From M~0.1 to M~0.5 Msolar, median LX/Lbol values increase by about half an order of magnitude and then remain constant at ~10-3.5 for the mass range from 0.5 to 3.0 M/Msolar. In these same two mass ranges, LX/Lbol remains roughly constant with age, until it drops by more than 2 orders of magnitudes at the epoch when ~2-4 Msolar stars are expected to become fully radiative. We find a dependence of LX and LX/Lbol on circumstellar accretion indicators and suggest three possible hypotheses for its origin. In spite of improved X-ray and rotational data, correlations between activity indicators and rotation remain elusive for these stars, possibly indicating that stars for which rotational periods have been measured have reached some saturation level. Our study of X-ray activity versus stellar mass leads us to propose that the few HRC X-ray sources not associated with any optical/infrared counterpart trace a yet-to-be-discovered stellar population of deeply embedded, relatively massive ONC members.
The Astrophysical Journal, Volume 582, Issue 1, pp. 398-409.Jan 2003 - Observational clues for a role of circumstellar accretion in PMS X-ray activity
Abstract
We revisit the published analyses of ROSAT X-ray observations of the star forming regions NGC 2264 and Chamaeleon I ( ~ 3 and ~ 5 Myr old respectively) in the light of newly published optical data. At odds with previous results on Chamaeleon I members, we find that low mass stars in both regions have near-saturated emission levels. Similarly to what previously found in the Orion Nebula Cluster, Weak Line T-Tauri Stars in NGC 2264 and in the Chamaeleon I cloud have higher X-ray activity levels respect to Classical T Tauri Stars, arguing in favor of a role of the disk and/or accretion in determining X-ray emission.
Astronomy and Astrophysics, v.397, p.611-621 (2003)Jan 2003 - The stellar content of the Chandra Deep Field North survey
Abstract
Not Available
Astronomische Nachrichten, Vol. 324, No. 1-2, p. 148n/a 2003 - The HELLAS2XMM survey: properties of the stellar population
Abstract
Not Available
Astronomische Nachrichten, Vol. 324, No. 1-2, p. 144n/a 2003
2002
- ROSAT PSPC/HRI observations of the open cluster NGC 2422
Abstract
We present the results of a ROSAT study of NGC 2422, a southern open cluster at a distance of about 470 pc, with an age close to the Pleiades. Source detection was performed on two observations, a 10-ks PSPC and a 40-ks HRI pointing, with a detection algorithm based on wavelet transforms, particularly suited to detecting faint sources in crowded fields. We have detected 78 sources, 13 of which were detected only with the HRI, and 37 detected only with the PSPC. For each source, we have computed the 0.2-2.0 keV X-ray flux. Using optical data from the literature and our own low-dispersion spectroscopic observations, we find candidate optical counterparts for 62 X-ray sources, with more than 80% of these counterparts being late type stars. We have assigned to the optical sources an astrometric and/or a photometric membership flag, depending on which data are available to us. The number of sources (38 of 62) with high membership probability counterparts is consistent with that expected for Galactic plane observations at our sensitivity. We have computed maximum likelihood X-ray luminosity functions (XLF) for F and early-G type stars with high membership probability. Heavy data censoring due to our limited sensitivity permits determination of only the high-luminosity tails of the XLFs; the distributions are indistinguishable from those of the nearly coeval Pleiades cluster.
Astronomy and Astrophysics, v.387, p.463-478 (2002)May 2002 - Discovery of X-ray emission from the protostellar jet L1551 IRS5 (HH 154)
Abstract
We have for the first time detected X-ray emission associated with a protostellar jet, on the jet emanating from the L1551 IRS5 protostar. The IRS5 protostar is hidden behind a very large absorbing column density, making the direct observation of the jet's emission possible. The observed X-ray emission is likely associated with the shock ``working surface'', i.e. the interface between the jet and the circumstellar medium. The X-ray luminosity emanating from the jet is, at LX =~ 3 x 1029 erg s-1, a significant fraction of the luminosity normally associated with the coronal emission from young stars. The spectrum of the X-ray emission is compatible with thermal emission from a hot plasma, with a temperature of =~ 4 MK, higher than the temperature expected (on the basis of the jet's velocity) for the shock front produced by the jet hitting the circumstellar medium.
Astronomy and Astrophysics, v.386, p.204-210 (2002)Apr 2002 - On X-ray variability in ROSAT-PSPC observations of F7-K2 stars
Abstract
We have analyzed the X-ray variability of dF7-dK2 stars in the solar neighborhood detected with the pointed ROSAT-PSPC observations. Our data base is the sample of all stars listed in the CNS3 catalog (Gliese & Jahreibeta 1991) having a B-V color between 0.5 and 0.9; it includes 70 pointed observations of 40 distinct stars or multiple systems. We have applied the unbinned Kolmogorov-Smirnov test on all X-ray photon time series of our sample: only 10 observations relative to 8 distinct stars are variable at a confidence level greater than 99% and 4 of them belong to multiple systems. For the subsample of 9 stars observed both at the beginning and at the end of the mission, we can study the variability on time scale of years and compare amplitude variations at short and long time scales. Our analysis suggests that, for these stars, the X-ray variability is more likely on longer time scale. All the stars variable on long time scale, and not on short time scale, are relatively quiet and similar to the Sun, suggesting that the variations may be due to cycles. The comparison of our results with those previously obtained for dM stars shows that the amplitude of variability of X-ray emission from dF7-dK2 stars is smaller than that observed in dM stars.
Astronomy and Astrophysics, v.383, p.210-217 (2002)Jan 2002
2001
- A ROSAT medium-sensitivity Galactic plane survey at 180°< l < 280°
Abstract
We have performed a moderately deep soft X-ray (0.1-2keV) survey of the Galactic plane using pointed observations with the ROSAT Position Sensitive Proportional Counter (PSPC). The survey is more than an order of magnitude more sensitive than previous X-ray surveys near the Galactic plane. The data consist of nine fields each of ~10ks exposure, pointed at positions on or very close to the Galactic plane (|b|<0°.3) in the longitude range 180°0.002counts-1. We present the catalogue of X-ray sources and the number-flux relations (logN-logS). Eighteen sources have possible identifications from the SIMBAD data base. We have searched the Tycho-2 and USNO-A2.0 catalogues to find all possible optical counterparts brighter than ~19th magnitude, and attempt to classify these on the basis of log(fX/fopt) versus optical colour diagrams and near-infrared photometry from the 2MASS Second Incremental Data Release. Hence, we have found the majority of these sources to be consistent with being late-type main-sequence stars, as previous studies have proposed from incompletely identified surveys. Comparison of the measured number-flux relations with predictions of Galactic (stellar) and extragalactic populations supports the view that the population of young stars in the plane is denser than previously thought.
Monthly Notices of the Royal Astronomical Society, Volume 326, Issue 3, pp. 1161-1182.Sep 2001 - Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests
Abstract
We present an extensive study on the dependence of the convective turnover time (tauconv ) on the stellar metallicity and age for main-sequence stars of masses 0.6-1.6 Msun. To this aim we have used and compared predictions by stellar models based on the classical Mixing Length Theory and models incorporating a Full Spectrum of Turbulence treatment of subphotospheric convection. We show that the metallicity effect is relevant for dG stars but negligible for dK stars, while stellar age is important when computing the turnover times for red dwarfs younger than log t ~ 8.5 yr. A scatter by up to a factor 3 could be spuriously introduced in the activity vs. Rossby number relationships if such effects are neglected. We have tested our model predictions on the metallicity effect by comparison with the observed median X-ray luminosities for dG and dK stars in the open clusters NGC 2516, the Pleiades, and Blanco 1, having similar age but likely different metal contents. We show that our model predictions, taking into account also the dependence of the coronal plasma emissivity on the metal abundance, are in agreement with the observations in all cases except for the dK stars in the metal-rich cluster Blanco 1, where our assumption of a rotation period distribution similar to those of the other cluster need to be confirmed by future observations.
Astronomy and Astrophysics, v.373, p.597-607 (2001)Jul 2001 - XRASE: the X-ray spectroscopic explorer
Abstract
Not Available
Astrophysics and Space Science, v. 276, p. 49-65 (2001).Mar 2001 - XRASE: the X-ray spectroscopic explorer
Abstract
Not Available
Astrophysics and Space Science, v. 276, p. 49-65 (2001).Mar 2001 - Chandra Observations of the Open Cluster NGC 2516
Abstract
Our analysis of Chandra X-Ray Observatory data for the open cluster NGC 2516, sometimes referred to as ``the southern Pleiades,'' has yielded over 150 X-ray detections in both High-Resolution Camera and Advanced CCD Imaging Spectrometer images of the central region of the cluster. We identify some of the new X-ray sources with photometric cluster members and compare these new Chandra results with those of ROSAT. To date, 82 detected X-ray sources (42% of surveyed cluster members) are tentatively identified as cluster members. We also discuss the X-ray properties of late-type members in comparison with those of corresponding stellar types in the more metal-rich, approximately coeval Pleiades Cluster.
The Astrophysical Journal, Volume 547, Issue 2, pp. L141-L145.Feb 2001 - XMM-Newton survey of the low-metallicity open cluster NGC 2516
Abstract
We present the first results of an XMM-Newton EPIC survey of NGC 2516, a southern low-metallicity open cluster with an age close to the Pleiades. The attained limiting sensitivity is of ~ 2.4 10-15 erg s-1 cm-2 in the 0.1-4.0 keV bandpass. This has been achieved by summing the data of the MOS and PN cameras of two distinct observations for a total exposure time of ~ 33 ks and by analyzing the summed data set with the wavelet detection algorithm developed at Osservatorio Astronomico di Palermo (OAPA) that has yielded over 200 X-ray detections. Using data just from a single exposure or from a single camera would have reduced by a factor 2-4 our limiting sensitivity and would have resulted in 25-40% less X-ray detections. To date, 129 detections have as counterparts one or more of the 540 photometrically selected cluster members in the surveyed region, for a total of 147 likely detected members, with unique identification in 112 cases. We derive the X-ray luminosity functions (XLF) of NGC 2516 members of different spectral types and compare them with those of the more metal rich, approximately coeval Pleiades cluster, finding the NGC 2516 photometrically selected dG and dK stars less luminous than the Pleiades. The XLFs of the NGC 2516 and of the Pleiades dM stars are indistinguishable. We compare the XMM-Newton results with those recently obtained with Chandra.
Astronomy and Astrophysics, v.365, p.L259-L266 (2001)Jan 2001
2000
- The structure of Algol’s corona: a consistent scenario for the X-ray and radio emission
Abstract
We present a systematic analysis of the four known large X-ray flares detected to date on the eclipsing binary system Algol, using an approach based on hydrodynamic simulations of decaying flaring loops including sustained heating. This method yields, for the large BeppoSAX Algol flare of Aug. 1997 (where a geometrical estimate of the size of the flaring region is available) a more reliable size than approaches based on the free decay of the flaring loop. For the three flares analyzed here (one observed by EXOSAT, one by GINGA and one by ROSAT) we show that indeed sustained heating is present in all cases, so that the size of the flaring region is always smaller than previously derived. No evidence for the very long loops previously found through quasi-static analysis methods (extending out to several stellar radii) is found. Instead, the flaring corona of Algol is found to be rather compact. By comparing the imaging VLBI observations of the radio corona of Algol with the recent location of the Algol flare seen by BeppoSAX and with with present results, a consistent model of the Algol corona is deduced: the corona is essentially concentrated onto the polar regions of the K star in Algol, with a more compact (smaller than the star) flaring component and a perhaps somewhat more extended (comparable {in size} to the star) quiescent corona.
Astronomy and Astrophysics, v.362, p.628-634 (2000)Oct 2000 - Evolution of X-ray activity of 1-3 Msun late-type stars in early post-main-sequence phases
Abstract
We have investigated the variation of coronal X-ray emission during early post-main-sequence phases for a sample of 120 late-type stars within 100 pc, and with estimated masses in the range 1-3 Msun, based on Hipparcos parallaxes and recent evolutionary models. These stars were observed with the ROSAT/PSPC, and the data processed with the Palermo-CfA pipeline, including detection and evaluation of X-ray fluxes (or upper limits) by means of a wavelet transform algorithm. We have studied the evolutionary history of X-ray luminosity and surface flux for stars in selected mass ranges, including stars with inactive A-type progenitors on the main sequence and lower mass solar-type stars. Our stellar sample suggests a trend of increasing X-ray emission level with age for stars with masses M > 1.5 Msun, and a decline for lower-mass stars. A similar behavior holds for the average coronal temperature, which follows a power-law correlation with the X-ray luminosity, independently of their mass and evolutionary state. We have also studied the relationship between X-ray luminosity and surface rotation rate for stars in the same mass ranges, and how this relationships departs from the Lx ~ vrot2 law followed by main-sequence stars. Our results are interpreted in terms of a magnetic dynamo whose efficiency depends on the stellar evolutionary state through the mass-dependent changes of the stellar internal structure, including the properties of envelope convection and the internal rotation profile.
Astronomy and Astrophysics, v.361, p.614-628 (2000)Sep 2000 - A BeppoSAX and ROSAT view of the RCW86 supernova remnant
Abstract
We present a spectral analysis of the Northern and Southwestern rim of the RCW86 X-ray shell, pointing out the remarkable differences between these two parts of the same object. In the North, a single temperature Non Equilibrium of Ionization emission model describes the data well, and the derived abundances of O, Ne, Mg, Si, S, Ar and Fe are in agreement with expected metal depletion behind a fast shock moving in the interstellar medium. If the initial explosion energy was ~ 1051 erg, the derived distance and age are 1.18-0.16+0.17 kpc and 1630-360+440 yr, fully consistent with the association to the historical supernova SN185. The X-ray emission from the SW is described by a two-temperature emission model with kTh = 5.7 keV and kTl = 1.0 keV. There is evidence of overabundant metals in the high-temperature component, thus indicating the presence of ejecta. In this case, a Type Ia SN is to be preferred over a more energetic Type II event, which would imply a more distant and older remnant fully in its Sedov phase.
Astronomy and Astrophysics, Vol. 360, p.671-682Aug 2000 - Multi-wavelength observations and modelling of shock-cloud interaction regions in the Vela Supernova Remnant
Abstract
We present optical photometric and spectrophotometric observations of two selected filaments near the North rim of the Vela supernova remnant also observed in UV with the International Ultraviolet Explorer, and in X-rays with the ROSAT PSPC detector. From the optical data, we have derived spatially-resolved ( ~ 3'') profiles of several emission lines across the filaments, which allowed us to resolve the post-shock region. Starting from model-independent line ratio diagnostics, we have estimated densities (/line{n}=3.2-8.2 cm-3), temperatures (T=3-10x 104 K), pressures (P=2.8x 10-10 dyne cm-2) and ambient magnetic fields (B=50-85 mu G) in the post-shock flow. We have also found clear evidence of a relative displacement between the X-ray emission, the emission in the Hα line and the emission in the [O III] line. We have compared the measured displacement, the width of the filaments in Hα and [O III], and the peak values of selected line ratios with the predictions of a radiative shock model which describes in detail the structure of the post-shock region. Shock models with velocities around v_s=110 km sec-1 are shown to be consistent with the optical and UV observations if we assume that the direction of propagation is tilted with respect to the line of sight. The layout of the X-ray and optical emission of the filament and their thermodynamical properties strongly suggest that the shock is travelling along increasing density gradient.
Astronomy and Astrophysics, v.359, p.316-336 (2000)Jul 2000 - ROSAT HRI survey of the open cluster Stock 2
Abstract
We have obtained two deep HRI observations toward the open cluster Stock 2 in order to assess the X-ray luminosity level of the 112 high-probability cluster members falling in the area we have surveyed. Members have been selected on the basis of the result of a recent proper motion study based on the same photographic material and the same reduction scheme as the GSC-II project. We have reached a limiting sensitivity of L_X ~ 3 x 1029 erg/sec for Stock 2 members and have detected 19 X-ray sources. The attained limiting sensitivity has prevented a complete sampling of the coronal emission level of Stock 2, and we have detected only three solar-type and one B9V members. We have derived the X-ray luminosity function for the Stock 2 stars in the 0.43 < (B-V)_o < 0.8 color range and have compared it with the X-ray luminosity functions of the dG members of Praesepe, Hyades and Pleiades open clusters. While the limiting sensitivity of our observations and the present uncertainties in Stock 2 and other cluster distances do not allow to derive firm conclusion, yet, our data indicate that the coronal emission level of Stock 2 solar-type stars is more similar to that of the Hyades (or the Praesepe) than to the Pleiades one. This in turn suggests that either Stock 2 is older than previously reported or that the current age-rotation-activity paradigm need to be revised. In this latter case Stock 2 would likely belong to the small group of open clusters (Praesepe, NGC6333 and IC 4675) for which recent X-ray surveys indicate a more complex age-rotation-activity relation than currently thought.
Astronomy and Astrophysics, v.357, p.460-470 (2000)May 2000 - Rosat HRI observations of the open cluster NGC 2516
Abstract
We present ROSAT HRI observations of the NGC 2516, a young southern open cluster with low metallicity. We detect 73 X-ray sources already detected with the PSPC and 12 new X-ray sources. The higher HRI spatial resolution allows us to improve the position of the previous known sources, and to better determine the optical counterparts of X-ray sources with respect to the PSPC data. We find 28 sources with no catalogued counterparts, most with faint objects within their X-ray positional error circle. Multiband CCD photometry of these possible counterparts shows that about half of them are consistent with being dK and dM cluster members. Assuming that they are true NGC 2516 members, the derived X-ray luminosity is consistent with that observed in dK and dM stars in more metal-rich open clusters of similar age, suggesting that the activity level at which X-ray saturation occurs is insensitive to a change of a factor two in stellar metallicity.
Astronomy and Astrophysics, v.357, p.909-919 (2000)May 2000 - HRI observations of PMS stars in NGC 2264
Abstract
We analyze six ROSAT HRI observations pointed toward the Star Forming Region (SFR) NGC 2264. Three are pointed to the southern star formation core, the other three about 20' to the north. We detect 169 X-ray sources, ~ 95% of which are likely to be Pre Main Sequence (PMS) stars, significantly enlarging the known population of the SFR in the area covered by the observations. Using published BVRI photometry we place the X-ray sources with well defined optical counterparts on the HRI diagram and estimate their masses and ages. Our comparison of the mass function and age distribution of the X-ray sources with results previously obtained for NGC 2264, demonstrates that deep X-ray observations provide, at least in this case, a very efficient method of selecting SFR members and does not introduce stronger biases than other methods. Since the observation cover a time span of ~ 5 years, we are able to study in detail the X-ray variability of our sample of PMS stars. We find that: 1) a large fraction of our sources are variable on several time scales and 2) Classical T Tauri Systems (i.e. stars surrounded by disks) are significantly more variable than the rest of our sample, suggesting a role of accretion disks in the emission and/or in the absorption of the X-ray radiation.
Astronomy and Astrophysics, v.355, p.651-667 (2000)Mar 2000 - An extreme X-ray flare observed on EV Lac by ASCA in July 1998
Abstract
We present a long (150 ks elapsed time) X-ray observation of the dM3.5e star EV Lac, performed with the ASCA observatory in July 1998, during which an exceptionally intense flaring event (lasting approximately 12 ks) was observed: at the flare's peak, the X-ray count rate in the ASCA GIS detectors was =~ 300 times the quiescent value. The physical parameters of the flaring region have been derived by analyzing the decay, using both a ``classic'' quasi-static approach and an approach based on hydrodynamic simulations of decaying flaring loops. Notwithstanding the large peak X-ray luminosity, this second method shows that the flare's decay is compatible with its being produced in a relatively compact region of semi-length L =~ 1.3 x 1010 cm ( =~ 0.5 R_*), large but not exceptional even by solar standards. The flare decays is fast (with a measured e-folding time for the light curve of <= 2 ks), but nevertheless the hydrodynamic-based analysis shows strong evidence for sustained heating, with the shape of the light curve dominated by the time evolution of the heating rather than by the natural cooling of the flaring plasma. As a consequence, the quasi-static method yields a much larger estimate of the loop's length (L =~ 2 R_*). The event shows (similarly to some other well-studied large stellar flares) a light curve characterized by two separate decay time constants (with the initial decay being faster) and a significant enhancement in the plasma metal abundance at the peak of the flare. The energetics of the event are exceptional, with the peak X-ray luminosity of the event reaching up to =~ 25% of the bolometric luminosity of the star, making this one of the largest X-ray flare (in relative terms) observed to date on a main-sequence star.
Astronomy and Astrophysics, v.353, p.987-997 (2000)Jan 2000
1999
- BVRI photometry of the star-forming region NGC 2264: the initial mass function and star-forming rate
Abstract
The pre-main sequence (PMS) population in the mass range between ~ 0.2 and ~ 3 M_sun in southern part of the star-forming region NGC 2264 has been studied, determining both the Initial Mass Function in the region as well as the star-formation rates for different mass ranges. The sample is a composite one, derived through the union of samples obtained through different techniques and each suffering from different biases: previously known PMS stars in the region from the literature, photometrically-selected T Tauri candidates (from our own photometric data, discussed in detail in the present paper) and X-ray selected PMS candidates (discussed in detail in a companion paper) have been joined to form a sample which we show to be statistically complete (i.e. free from the biases which affect each of the parent samples) down to =~ 0.6 M_sun (while being incomplete at lower masses). Individual masses and ages have been derived by placing the individual stars on evolutionary tracks, allowing us to derive both the IMF and the star formation rate. The Initial Mass Function thus derived for NGC 2264 shows evidence for a bimodal distribution of masses, with a break in the IMF at around 1 M_sun. Based on observations with the VATT: the Alice P. Lennon Telescope and the Thomas J. Bannan Astrophysics Facility. Table~3 is available in electronic form at CDS via ftp 130.79.128.5
Astronomy and Astrophysics, v.345, p.521-530 (1999)May 1999 - X-ray observations of the young open cluster Blanco 1
Abstract
We present results from two deep ROSAT HRI exposures on the Blanco 1 open cluster. Blanco 1 is one of the few open clusters at large distance from the Galactic Plane. This circumstance has suggested that it may have formed through a mechanism different from that responsible for the other nearby clusters. The age of Blanco 1 is rather uncertain since, depending on the adopted age indicator, it ranges between 30 and 90 Myr. Many cluster members show chromospheric emission typical of young stars. The X-ray observations presented here reveal a high number of X-ray sources, consistent with the young age of the cluster. The typical X-ray luminosity of the cluster members is consistent with that of the Pleiades and significantly lower than the alpha Per members. These results suggest than the age of Blanco 1 is more similar to the age of the Pleiades than to that of alpha Per. The time sampling of the X-ray observations allows us to study the variability of the sources on time scales from hours to 6 months - 1 year.
Astronomy and Astrophysics, v.344, p.83-96 (1999)Apr 1999 - ROSAT High Resolution Imager Identifications of Suspected Stellar Sources from the Einstein Slew Survey
Abstract
We present analysis of a series of ROSAT High Resolution Imager (HRI) observations of 16 X-ray detections from the Einstein Slew Survey (ESS). The data were taken to resolve ambiguous ESS identifications based on optical spectroscopy. For 10 of the 16 detections, the previous identification has been confirmed-six with solar-type stars (spectral types F-K), one each with a cataclysmic variable, a B2 V, a dMe star, and one with a BL Lac object. For another detection, two potential counterparts are unresolvable even with the HRI. The HRI does resolve two other ESS detections into two distinct X-ray sources, but no HRI source is found for the remaining three X-ray detections. HRI-derived fluxes are consistent with those measured by the ROSAT All-Sky Survey for their 12 common detections, and improved positions are obtained for these sources. We calculate values of f_X/f_V and L_X for these stars where possible and find values consistent with those expected for corresponding spectral types.
The Astronomical Journal, Volume 117, Issue 4, pp. 1845-1851.Apr 1999 - ROSAT PSPC observation of the NE region of the VELA supernova remnant. III. The two-component nature of the X-ray emission and its implications on the ISM
Abstract
We reconsider two ROSAT observations of the Vela SNR shock region, with the aim to overcome the problems encountered in interpreting the X-ray spectra in terms of single-temperature NEI emission model. We pursue the idea of the association of the observed cool and hot components with an high-density (``inhomogeneities") and a low-density (``intercloud") ISM phase, respectively. We derive the density (0.5-1.0 cm(-3) ) and the volume filling factor (0.2-0.8) of the inhomogeneities on a scale length of 10' ( ~ 1 pc), and we identify two regions of the main shock interactions, one with a small ( ~ 1 pc) isolated cloud, and the other with a larger feature ( ~ 4-5 pc, perhaps a cavity wall). We argue that the thermal evaporation from shocked clouds may account for the mass of observed inhomogeneities. After isolating the emission of the dynamically unperturbed intercloud medium, we apply the Sedov analysis and estimate the explosion energy of the supernova event (E_0 ~ 1.2x 10(50) erg) and the distance ( ~ 280 pc, in agreement with recent results). Our data also indicate fast electron-ion equipartition behind the shock. The influence of Non-Equilibrium of Ionization effects on our conclusions are estimated.
Astronomy and Astrophysics, v.342, p.839-853 (1999)Feb 1999 - X-ray spectroscopy of the active dM stars: AD Leo and EV Lac
Abstract
We present results of an analysis of new observations of the active M dwarfs AD Leo (GJ 388) and EV Lac (GJ 873), performed with the X-ray satellite SAX, and compare them with both published and new analyses of ROSAT PSPC observations of these stars. The PSPC spectra can be fitted with one- (EV Lac) or two-component (AD Leo) isothermal mekal models, and very low metallicity ( ~ 0.1 solar); we have found clear evidence of spectral variations in correspondence of a large flare observed during one of the PSPC observations of EV Lac, requiring the addition of a second component with log T ~ 7.5 to fit the flare spectrum. The SAX light-curves of AD Leo and EV Lac also show the occurrence of several flares. A two-component model does not provide an adeguate fit of the SAX spectra, regardless of the value of coronal metallicity. These spectra require at least three thermal mekal components and best-fit coronal plasma metallicity below solar for AD Leo and only marginally below solar for EV Lac, with 90% confidence ranges on Z/Z_sun = 0.22-0.35 and 0.36-1.11, respectively. We have also fitted the SAX spectra of AD Leo and EV Lac with model spectra from constant cross-section static coronal loops. One-loop models fail to fit the observed spectra. A second loop component, that accounts for most of the plasma emission at high energy, is required to obtain a fit of statistical quality just slightly worse than the 3-T fits. In the case of EV Lac both the 2-loop and the 3-T fits are unable to reproduce the observed emission below 0.5 keV. The available evidence points toward the existence of various (at least two) main classes of coronal emitting structures: the dominant one is composed of hundreds of compact loops, with relatively low maximum temperature and length smaller than 0.1 the stellar radius, covering no more than 1% of stellar surface; the second class, responsible for the high energy emission, is composed at least of tens of quite elongated loops, covering a very small fraction of stellar surface. We find no-evidence of loops with length comparable to the stellar radius.
Astronomy and Astrophysics, v.342, p.502-514 (1999)Feb 1999 - BeppoSAX and ROSAT observations of the supernova remnant RCW86
Abstract
We present the first results of a BeppoSAX observation of RCW86, covering the south-west part of the remnant (the knee). The spatial analysis of the LECS and MECS data confirms that strong variations of the emission hardness exist between the inner and the outer part of the knee. ROSAT data suggest that these variations are not due to absorption effects. The Non Equilibrium of Ionization (NEI) spectral analysis of the LECS data shows that the emission is characterized by the presence of a temperature component at kT~2keV, which, if compared with recent ASCA results obtained with a similar analysis, is softer, more absorbed, and not heavily metal-depleted. The LECS data also do not exclude the presence of a second very soft component (VSC) with kT~0.4keV. The VSC, if real, is to be associated with interaction of main blast wave with interstellar medium (ISM) inhomogeneities. The NEI spectral analysis of the ROSAT PSPC data confirms the presence of the VSC, and yields an estimate of the interstellar absorption and ionization parameters, but it does not exclude also higher temperature components.
Nuclear Physics B Proceedings Supplements, Volume 69, Issue 1-3, p. 92-95.Jan 1999 - Deep ROSAT HRI observations of the Pleiades
Abstract
In a deep X-ray survey of the Pleiades open cluster, we use the ROSAT High Resolution Imager to explore a region of the cluster formerly surveyed with the PSPC. These new observations substantially improve upon both the sensitivity and the spatial resolution for this region of the Pleiades, allowing us to detect 18 cluster members not detected before and 16 members not included in the catalogs used in previous surveys. The high sensitivity of the present observations permits us to obtain more stringent upper limits for 72 additional members and also provides sufficient numbers of stars to enable us to explore the dependence of L_x on stellar rotation for the slow rotators of the Pleiades. Using the new high sensitivity X-ray observations and the recent rotational measurements we discuss the activity-rotation relationship in the Pleiades solar type stars. We also present new photometric observations of optical counterparts of a number of X-ray sources detected in previous surveys but not yet identified. Table~2 is available in electronic form at CDS via ftp 130.79.128.5
Astronomy and Astrophysics, v.341, p.751-767 (1999)Jan 1999
1998
- X-ray spectroscopy of the weak-lined T Tauri star HD 283572
Abstract
We have analyzed the existing X-ray spectroscopic data for the Weak-Line T Tauri star (WTTS) HD 283572, from the ROSAT, ASCA and SAX satellites. All the data sets are well fit with an absorbed two-temperature model based on collisional ionization equilibrium. Together with the Einstein data discussed in the literature, these data sets afford a 16.5 years time span, allowing to study the short- and long-term spectroscopic and photometric variability characteristics of the X-ray emission from WTT stars. The X-ray luminosity of HD 283572 appears to vary by factors of =~ 2 on time scales of tens of kiloseconds, within a single observations, while its average X-ray luminosity has varied by a factor of =~ 2.5 (peak-to-peak) along the 16.5 yr studied here. The SAX observation also shows evidence for a short flare, with an enhancement of the X-ray luminosity by a factor of =~ 4.
Astronomy and Astrophysics, v.337, p.413-420 (1998)Sep 1998 - The evolutionary status of activity-selected solar-type stars and of T Tauri stars as derived from HIPPARCOS parallaxes: evidence for long-lived T Tauri disks?
Abstract
We have used the Hipparcos parallaxes to study the evolutionary status of a sample of stars with spectral types from late F to M0 (hereafter ``solar-type stars''), selected on the basis of their activity, mainly from Einstein-based surveys. The parallaxes have been used to place the objects in the H-R diagram, determining their age by comparison with theoretical evolutionary tracks and observational main sequences. This age is compared with age estimates derived from the lithium abundance, the activity level and the presence of circumstellar disks. To complement our sample at the young end we have also studied the Hipparcos-determined distances of a sample of optically-selected pre-main sequence stars, mostly classical T Tauri stars (CTTS). Some CTTS appear to be much nearer to us than previously determined, and far away from their putative parent cloud. This implies a significantly larger age providing observational evidence for the existence of long-lived T Tauri disks which could produce slow rotators on the Zero-Age Main Sequence (ZAMS). None of the above-mentioned age proxies appears to reliably and unambiguously select very young stars in the range of spectral types considered here, with some apparently very young objects effectively lying onto or very close to the main sequence. The attribution of ages to young solar-type stars on the basis of any of the standard proxies may thus significantly under- or over-estimate the evolutionary age of the object. Caution must therefore be exercised when attributing ages to individual stars, and claims about the large number of PMS stars found in X-ray based surveys may need to be at least in part reconsidered in this light. Based on data from the ESA Hipparcos satellite.
Astronomy and Astrophysics, v.335, p.218-226 (1998)Jul 1998 - An optimized time screening algorithm for ROSAT PSPC and HRI observations
Abstract
We have developed a model-independent time screening optimization algorithm to cope with significant contamination spikes in the ROSAT PSPC/HRI bacground light-curves. The rejection criteria are based on the maximization of faint sources signal-to-noise ratio. The algorithm tuning parameters have been optimized through performing a wide set of runs on both simulated and real data. We have verified that the application of our selection criteria to the case of long exposure PSPC observations yields an increase of the number of faint sources (SNR <6 sigma ) of up to 100% with a rejection of up to the 8% of the exposure time. At the same time, we obtain an average signal-to-noise ratio gain of 3% for the sources detected in both the unscreened and screened images. The algorithm, even though currently optimized for the instrumentation on board ROSAT, can be easily generalized for data analysis of other imaging X-ray detectors.
Astronomy and Astrophysics Supplement, v.129, p.647-655May 1998 - An X-ray study of the PMS population of the Upper Sco-Cen association
Abstract
We have searched for low-mass pre-main-sequence stars in the nearby Upper Sco-Cen star-forming region using ROSAT observations analyzed with an innovative source detection method based on wavelet transforms. In a 5.2 ksec ROSAT PSPC image we find 32 sources down to a limiting flux of 3.9 10(-14) erg cm(-2) s(-1) . In two HRI observations that cover with greater sensitivity approximately one-third of the same region we find 17 sources, down to a limiting flux of 1.4 10(-14) erg cm(-2) s(-1) . Half of the HRI sources were not detected either in the PSPC pointed observation nor are they reported in the ROSAT Bright Source Catalog, nor in the final analysis of Einstein IPC data of the same region, which resulted in the detection of 18 sources down to a limiting flux of 9.1 10(-14) erg cm(-2) s(-1) . Considering all the available X-ray data together, we have a total of 50 distinct X-ray sources, of which only 13 were previously reported as Einstein IPC sources in a sky region of nearly the same area as that surveyed in the present work. Our new analysis allows us to largely increase the census of likely PMS stars in the Upper Sco-Cen region and shows that the IPC survey of PMS stars in this region by Walter and collaborators is more severely incomplete than previously reported. The likely PMS nature of the stellar counterparts of the faint X-ray sources is supported by follow-up optical spectroscopy of some of these sources, as well as by the presence of variability of X-ray emission in a substantial fraction of them, and by the characteristics of available X-ray spectral information.
Astronomy and Astrophysics, v.332, p.825-841 (1998)Apr 1998 - X-ray spectroscopy of the active giant beta Ceti: the SAX LECS view
Abstract
We present an X-ray observation of the active giant beta Cet, performed with the Low Energy Concentrator Spectrometer (LECS) on-board the SAX satellite. The resulting X-ray spectrum is well fit by an optically-thin plasma model with two discrete temperatures, and the inferred coronal metallicity is compatible both with the abundances derived from a re-analysis of the ASCA SIS spectrum of the same object, adopting the same plasma emission code (mekal), and with accurate photospheric abundances recently reported in the literature. The remarkable similarity between the X-ray spectrum of beta Cet and the one of the active binary Capella, also observed by SAX, rises again the issue of how some putative He-burning clump giants, like beta Cet, manage to retain (or regain) a relatively high X-ray activity level. Some implications of the new Hipparcos parallaxes, relevant for the coronal properties of beta Cet and the Hyades giants, are also discussed.
Astronomy and Astrophysics, v.330, p.139-144 (1998)Feb 1998 - X-ray Variability and Rotation in the Pleiades Cluster
Abstract
Not Available
Astrophysics and Space Science, v. 261, Issue 1/4, p. 105-106 (1998).n/a 1998 - A Deep X-ray Survey of the PMS Population of the Upper Sco-Cen Association
Abstract
Not Available
Astrophysics and Space Science, v. 261, Issue 1/4, p. 121-122 (1998).n/a 1998 - An extended galactic plane survey of the third quadrant (180(deg) < L < 280(deg) )
Abstract
Poster paper
Astronomische Nachrichten, vol. 319, no. 1, Issue 1, p. 108.n/a 1998 - Problems and solutions in deriving the logN–logS of sources detected in PSPC images with a wavelet-transform algorithm
Abstract
Poster paper
Astronomische Nachrichten, vol. 319, no. 1, Issue 1, p. 78.n/a 1998
1997
- On coronal abundances derived with the SAX/LECS and ASCA/SIS detectors.
Abstract
We have studied the performance of global χ^2^ fitting of low-resolution X-ray spectra in retrieving intrinsic source parameters, with emphasis on the coronal metallicity. The study has been conducted by fitting large numbers of simulated spectra with known characteristics, and studying the distribution of best-fit parameters. We have studied the behavior of the LECS detector on board the SAX satellite and the SIS detector on board the ASCA satellite. The fitted source spectra have either two discrete temperature components or a power-law temperature distribution, with metallicity variations modeled by a single global abundance parameter. The model used for the fitting has always been a two-temperature one, with global varying abundance, to explore the influence of the a priori ignorance of the actual temperature stratification in the source being observed. The simulations performed explore the influence of varying statistics in the observed spectrum (spanning a realistic range of values) as well as the effect of varying the intrinsic source metallicity, with values in the range 0.15-1.0 times the solar value. We find that the source metallicity can be retrieved to within a few tens of percent from ASCA/SIS spectra of typical signal to noise ratio, and within few percent from SAX/LECS spectra at the same signal to noise ratio. However, relatively small uncertainties in the detector calibrations and in the plasma emission codes are likely to potentially cause large systematic off-sets in the value of the best-fit parameters. Similar systematic off-sets may derive from assuming too simplistic a temperature distribution for the source plasma. In addition we have re-analyzed the ASCA/SIS spectra of the active giants β Cet and Capella with the same set of assumptions used in the simulations, showing how the best-fit metallicity in these two real cases depends on the details of the fitting process, and in particular on the chosen energy range.
Astronomy and Astrophysics, v.326, p.1013-1022Oct 1997 - HIPPARCOS distances of X-ray selected stars: implications on their nature as stellar population.
Abstract
We present the parallaxes, measured by Hipparcos, for a sample of X-ray selected stars. The stars belong to the stellar sample of the Einstein Extended Medium Sensitivity Survey. They are all at galactic latitude |b|>20deg, and are generally far away from known star forming regions. Several of these stars show lithium abundance and activity level typical of very young stars with ages comparable to that of the Pleiades. We show that the majority of our sample stars are on the main sequence, with only =~20% being giants. We do not find a significant presence of pre-main sequence stars in our sample, notwithstanding the fact that some of our stars have a considerable lithium abundance, showing that the stars observed are most likely young and active main-sequence objects.
Astronomy and Astrophysics, v.326, p.221-227Oct 1997 - On the widespread Weak-Line T-Tauri population detected in the ROSAT All-Sky Survey.
Abstract
We discuss the apparent widespread presence of Weak-Line T-Tauri stars (WTTS) among stellar coronal sources detected in the ROSAT All-Sky Survey (RASS), and their relative number with respect to young main-sequence stars in the same samples. The approach taken in most of the current literature for identifying and classifying WTT stars among RASS X-ray sources is based on the usage of low-resolution optical spectra only and on simple, mass-independent thresholds on the equivalent width of the LiI 6707.8Å doublet. We show that this approach is likely to lead to putative WTTS samples which contain a large number of normal, young main-sequence stars masquerading as WTTS sources. Young main-sequence stars are known to be the dominant contributor in stellar X-ray selected samples at the limiting flux levels of the RASS, yet they appear to be very rare in the RASS samples discussed here. We argue that many of the putative WTTS sources are actually mis-classified young main-sequence stars, and that thus there is likely not a true ``WTTS question'' in the RASS samples.
Astronomy and Astrophysics, v.326, p.647-654Oct 1997 - The X-ray coronal emission of λ Andromedae observed with ASCA and ROSAT.
Abstract
We present an observation of the long-period RS CVn binary λ And obtained with ASCA. We also discuss a previous observation by ROSAT of the same star, that we have retrieved from the public archive. There was little variation in the quiescent X-ray flux of λ And between these two observations almost 5 years apart, but they offer different, and in part complementary, views of its corona. The ASCA spectra of λ And are well described by a 2-T model with a main component at T_1_=~0.9keV and a smaller component at T_2_=~3keV, with EM_2_/EM_1_=~0.15. The fitted coronal metallicity is Z=~0.1, much lower than solar but consistent within a factor of 2 with the reported photospheric metallicity Z_phot_=~0.2. On the contrary, a successful 2-T fit of the ROSAT spectrum yields a cool component at =~0.5keV and a hotter component at =~1.4keV, with EM_2_/EM_1_=~2 and Z=~0.25. Because of these differences, we were unable to find an acceptable fit to the joint ASCA and ROSAT data. We discuss possible causes for the discrepancy, including spectral time variability, uncertainties in the plasma emission codes and in the instrumental calibration, and limitations in the adopted fitting model. The hydrogen column density toward λ And, well constrained by ROSAT, is N_H_=~1x10^19^cm^-2^, a factor ~2 larger than estimated from UV observations from Copernicus and the Hubble Space Telescope. Albeit small, this difference may indicate the presence of substantial ionization of the interstellar medium.
Astronomy and Astrophysics, v.325, p.664-676Sep 1997 - On the comparison between spectroscopic and photometric metallicity measurements in active solar-type stars.
Abstract
We present spectroscopic determinations of the Fe abundance in a sample of K-type stars selected from the X-ray flux-limited Einstein Extended Medium Sensitivity Survey. These stars have been shown to have a low value of the m_1_ Stroemgren photometric index implying low metal abundances, similar to the abundance observed in Pop. II stars. Yet these cool stars are not expected to be metal-poor, being most likely young and thus metal-rich. Based on an equivalent width-based abundance analysis which uses high resolution spectra in the region around the Li I 6707.8Å doublet, we show that none of the stars investigated shows any evidence for low metal abundances. Rather, all have solar or even slightly super-solar abundances. Thus, for very active K stars, photometric metallicity determinations appear to be strongly biased, with the stars appearing to be of much lower metallicity than they actually are. The stars in our sample are estimated to be coeval or younger than those of the Pleiades. If, as we argue, the bias in the photometric indices depends purely on the high activity level of these stars, it is then likely that all stars sufficiently young to have similar activity levels would display similar photometric anomalies. We then predict that a non-negligible fraction of the disk population K-type stars are expected to show comparable anomalies in their photometric indices. These stars will masquerade as Pop. II stars in photometric surveys, and significantly contaminate Pop. II star samples selected on the basis of photometric properties alone, down to quite faint magnitudes.
Astronomy and Astrophysics, v.324, p.998-1004Aug 1997 - The [Fe/H] distribution of a volume limited sample of solar-type stars and its implications for galactic chemical evolution.
Abstract
We present [Fe/H] determinations for a volume-limited sample of G and K dwarfs from the Gliese catalog of nearby stars. The [Fe/H] values were determined from the FeI lines present in =~40Å wide high resolution (R >=50000) spectra taken around the LiI 6707.8Å line, through an equivalent width analysis based on the latest Kurucz model atmospheres. The resulting abundance distribution, corrected for disk heating effects, is compared to recent models of galactic chemical evolution. The surprising result is that, while the abundance distribution of solar-type dwarfs hotter than =~5100K and the derived chemical evolution parameters (in particular the rate of Fe enrichment and the abundance spread at a given age) are compatible with previous determinations of the same quantities, dwarfs cooler than 5100K show a lack of metal-poor ([Fe/H]<~-0.4) objects, implying chemical evolution parameters (using the same model) not compatible with the ones derived for the hotter stars. Possible explanations for this fact are discussed, critically considering possible biases present in the parent sample, as well as each of the assumptions made in the chemical evolution model and in particular the assumption that the birth rate will be constant with time and independent of stellar mass.
Astronomy and Astrophysics, v.323, p.809-818Jul 1997 - A Method Based on Wavelet Transforms for Source Detection in Photon-counting Detector Images. I. Theory and General Properties
Abstract
We have developed a method based on wavelet transforms (WTs) to detect sources in astronomical images obtained with photon-counting detectors, such as X-ray images. The WT is a multiscale transform that is suitable for detection and analysis of interesting image features (sources) spanning a range of sizes. This property of the WT is particularly well suited to the case in which the point-spread function is strongly varying across the image, and it is also effective in the detection of extended sources. The method allows one to measure source count rates, sizes, and ellipticity, with their errors. Care has been taken in the assessment of thresholds for detection, in the WT space, at any desired confidence level, through a detailed semianalytical study of the statistical properties of noise in wavelet-transformed images. The method includes the use of exposure maps to handle sharp background gradients produced by a nonuniform exposure across the detector, which would otherwise yield many spurious detections. The same method is applied to evaluate upper limits to the count rate of undetected objects in the field of view, allowing a sensitivity map for each observation to be constructed.
The Astrophysical Journal, Volume 483, Issue 1, pp. 350-369.Jul 1997 - A Method Based on Wavelet Transforms for Source Detection in Photon-counting Detector Images. II. Application to ROSAT PSPC Images
Abstract
We apply to the specific case of images taken with the ROSAT PSPC detector our wavelet-based X-ray source detection algorithm presented in a companion paper. Such images are characterized by the presence of detector ``ribs,'' strongly varying point-spread function, and vignetting, so that their analysis provides a challenge for any detection algorithm. First, we apply the algorithm to simulated images of a flat background, as seen with the PSPC, in order to calibrate the number of spurious detections as a function of significance threshold and to ascertain that the spatial distribution of spurious detections is uniform, i.e., unaffected by the ribs; this goal was achieved using the exposure map in the detection procedure. Then, we analyze simulations of PSPC images with a realistic number of point sources; the results are used to determine the efficiency of source detection and the accuracy of output quantities such as source count rate, size, and position, upon a comparison with input source data. It turns out that sources with 10 photons or less may be confidently detected near the image center in medium-length (~104 s), background-limited PSPC exposures. The positions of sources detected near the image center (off-axis angles < 15') are accurate to within a few arcseconds. Output count rates and sizes are in agreement with the input quantities, within a factor of 2 in 90% of the cases. The errors on position, count rate, and size increase with off-axis angle and for detections of lower significance. We have also checked that the upper limits computed with our method are consistent with the count rates of undetected input sources. Finally, we have tested the algorithm by applying it on various actual PSPC images, among the most challenging for automated detection procedures (crowded fields, extended sources, and nonuniform diffuse emission). The performance of our method in these images is satisfactory and outperforms those of other current X-ray detection techniques, such as those employed to produce the MPE and WGA catalogs of PSPC sources, in terms of both detection reliability and efficiency. We have also investigated the theoretical limit for point-source detection, with the result that even sources with only 2-3 photons may be reliably detected using an efficient method in images with sufficiently high resolution and low background.
The Astrophysical Journal, Volume 483, Issue 1, pp. 370-389.Jul 1997 - The relationship between lithium and activity in disk population main sequence G and K stars.
Abstract
We present determinations of the lithium abundance (through observations of the LiI 6707.8Å line) in a sample of main sequence stars with spectral types ranging from late F down to M0, selected from the sample of nearby stars which have been observed with the Einstein observatory. We show that lithium abundance is correlated with X-ray luminosity for the cooler stars in our sample (4700>T_eff_>4000), while no correlation is present for the hotter stars. However, the correlation is almost completely driven by a very small number of high lithium, high X-ray luminosity (and thus plausibly very young) cool dwarfs present in the sample. A number of high lithium K dwarfs are present in the survey, and their characteristics and likely age are discussed in detail.
Astronomy and Astrophysics, v.322, p.131-138Jun 1997 - ROSAT PSPC Observation of the Northeast Region of the Vela Supernova Remnant. II. Spectral Analysis with a Nonequilibrium of Ionization Emission Model
Abstract
We have developed a grid of nonequilibrium of ionization (NEI) models that depends on the electron temperature and ionization time and that is suitable for use in fitting X-ray spectra. Using this grid, we have carried out a spatially resolved spectral analysis of a ROSAT Position Sensitive Proportional Counter (PSPC) observation of the northeast shock region of the Vela supernova remnant on an angular scale of 10' (~1.0 pc at the distance of the remnant). The adopted model passes the χ2 test at the 95% confidence level for 45% of the considered spatial bins, giving unexpectedly low values of the ionization time (log τ ~ 0-1 yr cm-3). We discuss the possibility that this result may be due to a reduced contribution from a line emission, which may be related to the locking of metals in grains. We also find significant temperature gradients on the scale explored, and we argue that they are correlated to interstellar matter inhomogeneities encountered by the shock front. The emission from the remaining 55% of the bins is incompatible with the single-tau single-temperature NEI model, and a feasible description rests on the two-temperature collisional equilibrium of ionization model we have adopted in our previous analysis of the same data set. We tentatively interpret our results in terms of the superposition of shocks propagating in an inhomogeneous medium, which produces a multitemperature and multi-ionization stage plasma.
The Astrophysical Journal, Volume 481, Issue 2, pp. 872-882.May 1997 - Coronal properties of nearby old disk and halo dM stars.
Abstract
ROSAT X-ray (PSPC) and EUV (WFC) observations of a sample of 12 nearby Halo and Old-Disk low mass stars have been analyzed to determine their emission levels, properties of their coronal spectra and to characterize their temporal variations on time scales from hours to ten years. The light curves of the old-disk stars of our sample with the highest count statistics, show that variability of a factor two on a time scale of a few thousands of seconds is a common property of old-disk stars. The coronal emission of GJ 191, the only halo star with enough counts to make the time analysis feasible, shows significant variations on a time scale of six months. The time resolved spectral analysis of the most intense stars shows that their coronal emission is consistent with a two temperature model with (approximately) constant values of the temperatures along the entire observation, while the observed variability can be explained by changes in their emission measure distributions. We find that the emission measures of the two components change coherently during the observations. For all ten detected stars we have computed hardness ratio and X-ray luminosity. While our old-disk stars fit well in the body of the distribution outlined by a complete volume limited sample of dM and dK stars within 7pc (Schmitt et al. 1995), the halo stars tend to occupy the softest and weakest envelope defined by population I low-mass stars.
Astronomy and Astrophysics, v.320, p.865-877Apr 1997
1996
- Stroemgren four-color photometry of X-ray active late-type stars: Evidence for activity-induced deficiency in the m_1_ index.
Abstract
We present the results of a uvby-β photometric study of a sample of active late-type stars (F-K) selected from the Einstein Extended Medium Sensitivity Survey. Our work shows the presence in the sample of a star population with the photometric index c_1_ typical of main sequence stars and an unexpected deficiency in m_1_ index. Stars with more anomalous values of m_1_ have also very high values of f_ X_/f_V_ and X-ray surface fluxes, near the "saturation" limit observed in the most active stars and similar to the flux observed in solar active regions. We discuss these results in the light of similar results found in the Sun comparing m_1_ indices in quiet and active regions and in other samples of active stars.
Astronomy and Astrophysics Supplement, v.119, p.403-412Nov 1996 - Lithium abundance in a volume-limited sample of nearby main sequence G and K stars.
Abstract
We present observations of the Lii 6707.8Angstroems line in a volume limited sample of main sequence stars of spectral types ranging from late F down to M0, selected from the Gliese catalog of nearby stars, discussing the behavior of lithium abundance along the main sequence. An unexpected result from our survey is that a low but measurable abundance of surface lithium (-0.5<N(Li)<-1.0) is found to be a common feature in the cooler dwarfs in our sample. Possible explanations for the widespread presence of lithium in very cool dwarfs are discussed, with emphasis on the possibility of lithium being produced in stellar flares. We also discuss the feasibility of using kinematic age indicators for studying the lithium-age relationship, and show that surface lithium is found to be essentially uncorrelated with kinematic age indicators for the dwarfs in our sample.
Astronomy and Astrophysics, v.311, p.951-960Jul 1996 - Optical and X-Ray Characteristics of Stars Detected in the Einstein Slew Survey
Abstract
We detect X-rays for the first time from 63 cool (types AM) stars. These stars are part of the 229 total stellar X-ray sources identified to date in the Einstein Slew Survey (hereafter Slew). We also list new X-ray data on one A star that may have a corona, five OB stars, and report discoveries of two new T Tauri stars and two new cataclysmic variables. The stellar content of the Slew high-latitude subset (currently 93% identified for |b_II_| > 20^deg^) is 26%. This agrees well with the Einstein Extended Medium Sensitivity Survey (EMSS) stellar sample, which only considered high latitudes. Because of the large solid angle covered by the Slew, and its shallow limiting flux, the sample will better probe the bright end of the stellar X-ray luminosity function (L_X_ > 10^30^ ergs s^-1^). Presently (based on the 221 Slew stars with known spectral types), the sample is dominated by late-type systems (cooler than F; 68%). These include dMe's, BY Dra, RS CVn, and FK Comae systems. Based on the limiting magnitudes of catalogs searched to date, the Slew is assessed to be complete for spectral types earlier than K. Hence, K and M systems will be prominent in the ~50 as yet unidentified stars. We have embarked on an extensive program to (1) confirm the X-ray identifications with the optical counterparts; (2) search for Ca II H and K and Balmer line emission as activity signatures; and (3) search for supporting evidence of magnetic activity by measuring rotational velocities and relating them to X-ray luminosity level. Of 64 proposed coronal systems observed to date, we have confirmed 44 active stars, while in the remaining 20 we have been unable to find definitive activity. We have confirmed an additional 19 active stars from stellar database searches, and reclassified nine systems as having nonstellar optical counterparts from optical follow-up identification work and extragalactic database searches. We discuss notable new discoveries. From the sample of single active F7-MS Slew stars with measured ν sin i-values, we find a strong (99.9% confidence level) linear correlation of X-ray luminosity with ν sin i and with stellar radius (R). However, L_X_ is uncorrelated with angular rotation speed at the 99% level. For the combined Slew and EMSS single star F7-MS sample, we find the same 99.9% ν sin i-L_X_ and R-L_X_ correlations. The L_X_-ν sin i relation for the combined sample appears to flatten with respect to the quadratic behavior seen for optically selected stellar samples at rotational velocities in excess of ~16 km s^-1^. For the unevolved subset of the Slew single star sample, we also find a correlation between L_X_ and Rossby number (R_0_; more than 99% confidence). A least-squares fit gives L_X_ ~ R_0_^-0.4^, which is similarly flatter than the quadratic dependence seen in optical samples. Using the stellar surface X-ray flux F_X_ versus B - V diagram, we interpret these results as saturation of the stellar surface by active regions at F_X_/F_bol_ ~ 10^-3^.
Astrophysical Journal v.463, p.747Jun 1996 - ROSAT Observations of the Pleiades. I. X-Ray Characteristics of a Coeval Stellar Population
Abstract
We report the results of a deep X-ray survey of the core region of the Pleiades carried out with ROSA T. In a single PSPC field (˜1° radius), we detect 99 of 214 Pleiades stars, and we compute upper limits for the remainder. We have detected all early dM stars that fell within 20' of the field's center, where the instrument's sensitivity is highest. We have computed maximum-likelihood integral X-ray luminosity functions in various B - V intervals and compared the results with previous studies of the Pleiades. Our results support the contention that B and dA star "detections" are likely due to X-ray emission from lower mass companions; we confirm the existence of a stellar age versus X-ray luminosity relation found with the Einstein Observatory for dG stars and extend it to dK and dM stars, and we construct an essentially statistically complete X-ray luminosity function for Pleiades dG stars and an almost complete X-ray luminosity function for dK stars. We have verified the stellar rotation versus X-ray luminosity relation for Pleiades dG stars and also show that a similar relation does not hold for dK stars. Indeed, it appears that some stellar parameters other than age and/or rotation rate must affect the level of X-ray emission of Pleiades dG and dK stars. We demonstrate the lack of measurable variability in X-ray emission at timescales of ˜6 months (except for flares) and the presence of variability by a factor of 2 on timescales of ˜10 yr in at least 15% of Pleiades members. The magnitude of this variability (by a factor of ˜2) is too small to account for the spread of the X-ray luminosity functions.
Astrophysical Journal Supplement v.102, p.75Jan 1996
1995
- The space density of active binaries from X-ray surveys.
Abstract
We review the currently available data on active binaries selected from Einstein observatory based X-ray surveys, and compare the inferred space density with the available estimates based on optically selected samples. It is shown that, in contrast to previous reports, no disagreement is present between the optical and X-ray based estimates, and that the space density of active binaries is likely to be smaller than previous estimates based on X-ray selected data. In particular, we show that of the 18 systems in the δ>-20deg subsample of the Einstein Extended Medium Sensitivity Survey previously classified as active binaries, 6 are not to be considered as such, while a further 5 have a somewhat doubtful status. The total space density we derive is 7.7x10^-5^pc^-3^ if the 5 doubtful systems are retained and 3.7x10^-5^pc^-3^ if they are excluded. Both values are substantially lower than the value of 4.5x10^-4^pc^-3^ originally derived from the same subsample of Extended Medium Sensitivity Survey fields. Also, the median X-ray luminosity of X-ray selected active binaries is found to be higher than previously reported, and again in contrast with previous reports, we find no disagreement between the X-ray luminosities of X-ray and optically selected samples. We briefly discuss the implication of the revised value of the space density on the contribution of active binaries to the soft X-ray background.
Astronomy and Astrophysics, v.298, p.482Jun 1995 - The discovery of a very active high proper motion dMe binary star.
Abstract
We present X-ray and optical observations of G102-21, a newly discovered dMe flare star in the solar neighborhood. The star, which falls serendipitously in a pointed PSPC ROSAT observation, shows an intense X-ray flare as well as other, lower-amplitude variations on time scales of around 10^4^ seconds. The decay time of about 1000sec, the characteristics of its X-ray spectrum, and the derived physical parameters, make the observed flare similar to compact solar flares and to many other flares observed on dMe stars, by previous missions. Low resolution spectroscopic observations show the whole Balmer series and the CaII H&K lines strongly in emission. Monitoring of the equivalent width of the Hα, Hβ and CaII H&K lines shows sizable night to night variations, with flaring-like behavior. Both Hα and H β lines show the presence of two components, which are very clearly separated at high resolution. The relative radial velocity changes from night to night, indicating a binary system. Based on simple assumptions we estimate that the system has a period of less than 1 day. The rotational broadening of the photospheric absorption lines observable in the high resolution spectra allows us to measure the stellar rotational velocity. The observed value (about 20km/s) is compatible with a tidally-locked system with a period between 0.5 and 1 day. The X-ray observations are consistent with a period in the range of 14-15 hours and with an inclination of the rotation axis near 50^o^.
Astronomy and Astrophysics, v.298, p.505Jun 1995 - An Objective Multicolor Method for the Characterization of Low-Resolution X-Ray Spectra
Abstract
We describe a new technique for the characterization of low-resolution X-ray spectra, based on generalized colors defined by means of a principal component analysis. Application of the method to a sample of optically and radio-selected Einstein Observatory X-ray sources yields groups that are dominated either by stars or by extragalactic sources. The method thereby provides an immediate interpretation in terms of differences in the physics of the emission processes dominating the objects studied. The principal advantages of the method are that it provides an objective means of classifying source spectra and that it provides an objective means of obtaining the minimal set of spectral classification parameters. Though the method does not significantly improve results obtained with previously employed techniques when applied to Einstein data, its objective operation should prove powerful in aiding source classification in the more recent medium spectral resolution X-ray surveys based on the ROSAT and ASCA mission.
Astrophysical Journal v.446, p.108Jun 1995 - Einstein Observations of T Tauri Stars in Taurus-Auriga. I. Properties of X-Ray Emission
Abstract
We present a systematic reanalysis of the Einstein IPC X-ray data on the pre-main-sequence stars in the Taurus-Auriga star formation region. We consider all cataloged stars in this region which have been observed with the IPC; of these, 52 out of 68 can be identified with X-ray sources; upper limits on the X-ray luminosity have been evaluated in the remaining cases. We derive an X-ray luminosity function and argue that its median Lx is representative of the population intrinsic value, in spite of incompleteness of the sample and possible ambiguities in the X-ray source identifications. As a result, it is confirmed that the known steady increase in the average level of X-ray emission toward younger ages can be extended to the pre-main-sequence phase. By using the line equivalent width as the relevant parameter, a statistically significant difference is found between the distributions of X-ray luminosities of stars with and without strong emission lines. No such difference is found instead between stars with and without strong infrared excess at 25 μm.
Astrophysical Journal v.446, p.331Jun 1995 - ROSAT PSPC spectral fitting simulations with one- and two-temperature models of optically-thin plasmas.
Abstract
We present results from an extensive number of PSPC spectral fitting simulations, performed with the aim to ascertain the dependence of the quality of the fit on photon counting statistics. In particular, we investigate the reliability of the results and their sensitivity to the assumed model in the case of X-ray emission from optically thin plasmas in thermal equilibrium, typical of stellar coronae and supernova remnants. We consider both single-temperature and two-temperature Raymond-Smith emission models, and the effects of contamination from either low or relatively high diffuse X-ray background. We explore the relative errors on the fitted temperatures derived from the simulations, and discuss constraints on the inferences that can be drawn regarding the existence of two components with different temperatures and emission measures in the observed spectrum. We also discuss the possibility that the presence of two components could be erroneously inferred from the analysis of a single-temperature spectrum. Moreover, we address the issue of the statistical independence of the spectral bins used in the fit by testing the agreement of the distribution of χ^2^ values obtained from the simulations with the theoretical distribution. Finally, we briefly investigate advantages and drawbacks of using the C statistics as an alternative to the χ^2^ statistics.
Astronomy and Astrophysics Supplement, v.110, p.573May 1995 - High resolution spectroscopy of old late K dwarfs stars around the lithium I 6707.8 A line: Is lithium there?
Abstract
We report recent high resolution, high signal to noise ratio spectroscopic observations of the region around the Li I 6707.8 A line in two apparently old field late K-type main sequence stars. These stars exhibit a small equivalent width line at the same wavelength, within 10 mA, as the Li I line. Lithium is expected to be fully depleted in old low mass stars, and therefore its presence in Hyades age (or older), 0.6 solar mass stars is surprising. Given the richness and complexity of the spectra in these cool stars, the identification of the line with the Li I line is not obvious. We briefly discuss, using model atmospheres and synthetic spectra alternative explanations for this feature. Finally, we briefly look at the implications of the presence of lithium in low mass stars for current stellar interior models.
Astronomy and Astrophysics, Vol. 297, p. L1-L4 (1995)May 1995 - The stellar coronal component of the Galaxy. II. an analysis of the stellar content of the Einstein Extended Medium Sensitivity Survey.
Abstract
We present a detailed analysis of the stellar content of the Einstein Extended Medium Sensitivity Survey (EMSS). The analysis consists of a comparison between the observed source counts and expectations computed on the basis of the known X-ray luminosity functions for disk population stars and established models for the structure of the Galaxy. We show that the presence of an excess of X-ray luminous "yellow" stars, already found with the analysis of the stellar content of the Einstein Medium Sensitivity Survey (MSS), is confirmed by the EMSS, whose substantially larger size sample allows to draw conclusions with a higher statistical significance than the MSS, and discuss the nature of this stellar population. Additionally, we show that to ensure the coherence between the observed number of dwarf M stars in the EMSS with the X-ray luminosity functions derived from optically selected sample, it is necessary to take into account the effects deriving from the variation of X-ray luminosity with stellar age, as well as the effect of stellar multiplicity.
Astronomy and Astrophysics, v.296, p.370Apr 1995 - Lithium, X-ray activity and rotation in an X-ray selected sample of solar-type stars
Abstract
We present an analysis of the connection between X-ray activity level, photospheric abundance of lithium and surface rotation in late type active main sequence stars (G and K), using the ratio between optical and X-ray luminosity fx/fv as an uniform activity indicator. We perform this analysis for a sample of X-ray selected sources from Einstein-based surveys compared to stars from the Pleiades open cluster and to a sample of active binary stars. We show that these parameters show different degrees of statistical correlation in the three samples. In particular, the Pleiades sample shows a significant correlation between all three quantities, while in the X-ray selected sample lithium and rotation are significantly correlated with each other but neither is correlated with the activity level. No correlation is evident for the three quantities studied in the active binary sample. We show how the behavior of the X-ray selected sample can be used to discriminate among different hypothesis about the nature of the so-called `yellow star excess' observed in X-ray flux-limited surveys, showing that this is composed by a population of young, near Zero-Age Main Sequence (ZAMS) stars with characteristics similar to the Pleiades.
Astronomy and Astrophysics, Vol. 295, p. 147-160 (1995)Mar 1995
1994
- Parallel computing on Unix workstation arrays
Abstract
We have tested arrays of general-purpose Unix workstations used as MIMD systems for massive parallel computations. In particular we have solved numerically a demanding test problem with a 2D hydrodynamic code, generally developed to study astrophysical flows, by exucuting it on arrays either of DECstations 5000/200 on Ethernet LAN, or of DECstations 3000/400, equipped with powerful Alpha processors, on FDDI LAN. The code is appropriate for data-domain decomposition, and we have used a library for parallelization previously developed in our Institute, and easily extended to work on Unix workstation arrays by using the PVM software toolset. We have compared the parallel efficiencies obtained on arrays of several processors to those obtained on a dedicated MIMD parallel system, namely a Meiko Computing Surface (CS-1), equipped with Intel i860 processors. We discuss the feasibility of using non-dedicated parallel systems and conclude that the convenience depends essentially on the size of the computational domain as compared to the relative processor power and network bandwidth. We point out that for future perspectives a parallel development of processor and network technology is important, and that the software still offers great opportunities of improvement, especially in terms of latency times in the message-passing protocols. In conditions of significant gain in terms of speedup, such workstation arrays represent a cost-effective approach to massive parallel computations.
Computer Physics Communications, Volume 83, Issue 2-3, p. 130-140.Dec 1994 - ROSAT PSPC observation of the northeast region of the VELA supernova remnant. 1: Evidence of thermal structure on a scale of 5 min
Abstract
We present the results of a 4 ks ROSAT position sensitive proportional counter (PSPC) observation pointed toward the NE region of the Vela supernova remnant (SNR). The image counting statistics allowed us to perform, for the first time, a spatially resolved spectral analysis of the X-ray emission on an angular scale of 5 min (approximately 0.5 pc assuming a distance D = 350 pc). Accounting for systematic uncertainties in the PSPC energy response, a simple one-temperature Raymond-Smith model yields an acceptable fit to the data in 69% of the spatial bins considered (out of 273), at the 95% confidence level. Using this model we discovered statistically significant variations of temperature (0.10-0.28 keV) and interstellar hydrogen column density (log NH = 19.5-20.5/sq cm) on the spatial scale explored. We interpret this result in terms of SNR shell expansion in a nonuniform medium. On the other hand, the data as a whole are locally best fitted by a two-temperature Raymond-Smith model, with T1 ranging between 0.05 and 0.22 keV, T2 in the range 0.2-0.9 keV, and with a ratio of emission measures EM2/EM1 less than or = 0.5. This result improves the previous single-temperature model description of the Vela SNR emission, based on lower spectral and spatial resolution Einstein data. We discuss the physical interpretation of the two-temperature model, suggesting that it can be explained by the under-ionization of the shocked plasma, as predicted by Itoh.
Astrophysical Journal v.437, p.209Dec 1994 - Einstein X-ray observations of Herbig Ae/Be stars
Abstract
We have investigated the X-ray emission from Herbig Ae/Be stars, using the full set of Einstein Imaging Proportional Counter (IPC) observations. Of a total of 31 observed Herbig stars, 11 are confidently identified with X-ray sources, with four additonal dubious identifications. We have used maximum likelihood luminosity functions to study the distribution of X-ray luminosity, and we find that Be stars are significantly brighter in X-rays than Ae stars and that their X-ray luminosity is independent of projected rotational velocity v sin i. The X-ray emission is instead correlated with stellar bolometric luminosity and with effective temperature, and also with the kinetic luminosity of the stellar wind. These results seem to exclude a solar-like origin for the X-ray emission, a possibility suggested by the most recent models of Herbig stars' structure, and suggest an analogy with the X-ray emission of O (and early B) stars. We also observe correlations between X-ray luminosity and the emission at 2.2 microns (K band) and 25 microns, which strengthen the case for X-ray emission of Herbig stars originating in their circumstellar envelopes.
Astrophysical Journal v.436, p.807Dec 1994 - ROSAT X-ray observations of late-type evolved stars: On the relationship between coronal temperatures and luminosities
Abstract
We present ROSAT Position Sensitive Proportional Counters (PSPC) X-ray observations of three near-solar-mass stars, in different evolutionary phases beyond the main sequence: eta Sco (F3 III-IV), iota Vir (F6 III), and HD 74772 (G5 III). All three of these nearby, presumably single stars have been detected, and we have collected enough counts to perform a detailed analysis of their soft X-ray spectra. While the X-ray spectra of eta Sco and HD 74772 can be fitted with Raymond-Smith thermal models with temperatures around 2 x 106 K, the high signal-to-noise spectrum of iota Vir provides unambiguous evidence of a multitemperature plasma, with a two-temperature best-fit model with components at approximately 2 x 106 K and 8 x 106 K. Evidence of some hot plasma (T approximately 107 K) has been also found for HD 74772. The present data, compared with spectral fitting results for other late-type stars observed with the Einstein Observatory, indicate that the low X-ray luminosity giants (Lx is less than 5 x 1028 ergs/s) do not share with the higher X-ray luminosity stars of the same class the property of having substantial amount of 107 K plasma. Moreover, our results confirm the trend of increasing X-ray luminosities with increasing coronal temperatures.
Astrophysical Journal v.432, p.701Sep 1994 - X-ray emission on hybird stars: ROSAT observations of alpha Trianguli Australis and IOTA Aurigae
Abstract
We report on deep ROSAT observations of two Hybrid atmosphere stars, alpha TrA and iota Aur, and our analysis of these observations. We detect high-energy transient phenomena on alpha TrA and consider the implications of this discovery to the atmospheres of Hybrid stars. We detect iota Aur in the high-energy passband of ROSAT, implying the existence of multimillion degree plasma on the star. Our major results include the following: discovery of two large flare events, detected during pointed observations of alpha TrA; the demonstration that the flare emission most likely comes from the giant itself, rather than from a previously unseen low-mass companion star; the demonstration that the plasma characteristics associated with the flares and with the 'quiescent' component are essentially indistinguishable; and that the geometric dimensions of the emitting plasma are considerably smaller than the critical dimension characterizing stable 'hot' coronal loop structures. Our results suggest that alpha TrA does not have any steady X-ray emission consistent with theoretical expectations, and support the argument that Hybrid stars constitute a transitional type of object in which large-scale magnetic dynamo activity ceases, and the dominant spatial scales characterizing coronal structure rapidly decline as such stars evolve across the X-ray 'Dividing Line' in the H-R diagram.
Astrophysical Journal v.431, p.402Aug 1994
1993
- A Search for Yellow Young Disk Population Stars among EMSS Stellar X-Ray Sources by Means of Lithium Abundance Determination
Abstract
Using previous radial velocity surveys of the EMSS stellar sample which have identified most of the detectable active binary systems, including X-ray luminous RS CVn type systems, we have selected a sample of apparently normal, mostly single EMSS stars with spectral type comprised between F5 and K8. We have studied this sample by means of high resolution spectral observations with the aim of measuring their photo- spheric Li abundances and, therefore, their age distribution. Our results support the hypothesis, based on a model of the stellar coronal component of the Galaxy, that, to be consistent with the accepted values for the stellar birthrate in the last billion years, a considerable fraction of the "normal" yellow stars detected in the EMSS must be young objects with ages comparable to the Pleiades.
Astronomy and Astrophysics, Vol. 277, p. 428-438 (1993)Oct 1993 - ROSAT X-Ray Detection of epsilon Taurus: Revisiting the Coronal and Transition Region Emission of the Hyades Giants
Abstract
We report on a recent pointed X-ray observation of the Hyades giant ɛ Tau, obtained with the ROSA T PS PC. We confirm at higher significance the earlier ROSAT all-sky survey detection of this star, with an X-ray luminosity of ˜1028 ergs s-1. ɛ Tau turns out to be the X-ray faintest among the four giants of the Hyades cluster, and the only one with no evidence of binarity. We rediscuss possible explanations, already put forward in previous studies, for the large spread in coronal and transition region emission observed among these stars. We revisit this issue in the light of our most recent knowledge on X-ray and UV emission properties of other Hyades and field stars.
Astrophysical Journal v.416, p.204Oct 1993 - X-ray emission at the low-mass end – Results from an extensive Einstein Observatory survey
Abstract
We have used available IPC data and a critical compilation of cataloged optical data to measure the 0.16-3.5 keV X-ray emission from 88 K and 169 M stars of luminosity classes IV, V, and VI within 25 pc from the Sun. The IPC detected 54 out of the 88 K stars, 70 out of the 138 M stars with M(v) less than 13.4, and 15 out of the 31 fainter M stars. We have identified a subsample of surveyed stars that is statistically representative of the population of K and M stars in the solar neighborhood. On the basis of this subsample (1) we have shown the occurrence of a drop in the level of X-ray emission for M stars later than approximately M5; (2) we have built unbiased maximum likelihood X-ray luminosity functions for the K, early M, and late M stars; (3) we have confirmed, both for K and M stars, the decrease of X-ray luminosity with increasing stellar age in the range of ages of disk population stars: and (4) we have shown that no obvious correlation is present between X-ray and bolometric luminosities in the entire representative samples of K and M stars, but only within flare stars which also seem to mark a saturation in X-ray luminosity level.
Astrophysical Journal v.414, p.846Sep 1993 - Stellar birthrate in the Galaxy – Constraints from X-ray flux-limited surveys
Abstract
The sensitivity of the stellar X-ray log (N) - log (S) to the relative density of young stars resulting from the assumed stellar birthrate is discussed. It is shown that given the sensitivity of X-ray model predicted log (N) - log (S) to the stellar birthrate, X-ray flux limited surveys can be used to impose relevant constraints on the history of the star-formation rate in the Galaxy. It is also shown that current X-ray surveys are compatible with high recent stellar birthrates, but incompatible with past stellar formation rates much higher than today's. The limitations of the present application due to available X-ray data are discussed.
Astrophysical Journal v.412, p.618Aug 1993
1992
- Fast parallel implementation of multidimensional data-domain FORTRAN codes on distributed-memory processor arrays
Abstract
We illustrate a general and straightforward approach to develop FORTRAN parallel two-dimensional data-domain applications on distributed-memory systems, such as those based on transputers. We have aimed at achieving flexibility for different processor topologies and processor numbers, non-homogeneous processor configurations and coarse load-balancing. We have assumed a master-slave architecture as basic programming model in the framework of a domain decomposition approach. After developing a library of high-level general network and communication routines, based on low-level system-dependent libraries, we have used it to parallelize some specific applications: an elementary 2-D code, useful as a pattern and guide for other more complex applications, and a 2-D hydrodynamic code for astrophysical studies. Code parallelization is achieved by splitting the original code into two independent codes, one for the master and the other for the slaves, and then by adding coordinated calls to network setting and message-passing routines into the programs. The parallel applications have been implemented on a Meiko Computing Surface hosted by a SUN 4 workstation and running CSTools software package. After the basic network and communication routines were developed, the task of parallelizing the 2-D hydrodynamic code took approximately 12 man hours. The parallel efficiency of the code ranges between 98% and 58% on arrays between 2 and 20 T800 transputers, on a relatively small computational mesh (≈3000 cells). Arrays consisting of a limited number of faster Intel i860 processors achieve a high parallel efficiency on large computational grids (> 10000 grid points) with performances in the class of minisupercomputers.
Computer Physics Communications, Volume 72, Issue 2-3, p. 129-143.Nov 1992 - An Einstein Observatory SAO-based catalog of B-type stars
Abstract
About 4000 X-ray images obtained with the Einstein Observatory are used to measure the 0.16-4.0 keV emission from 1545 B-type SAO stars falling in the about 10 percent of the sky surveyed with the IPC. Seventy-four detected X-ray sources with B-type stars are identified, and it is estimated that no more than 15 can be misidentified. Upper limits to the X-ray emission of the remaining stars are presented. In addition to summarizing the X-ray measurements and giving other relevant optical data, the present extensive catalog discusses the reduction process and analyzes selection effects associated with both SAO catalog completeness and IPC target selection procedures. It is concluded that X-ray emission, at the level of Lx not less than 10 exp 30 ergs/s, is quite common in B stars of early spectral types (B0-B3), regardless of luminosity class, but that emission, at the same level, becomes less common, or nonexistent, in later B-type stars.
Astrophysical Journal Supplement v.81, p.795Aug 1992 - Modeling the stellar contribution to the Galactic component of the diffuse soft X-ray background. I – Background fluxes and number counts
Abstract
A combination of a stellar Galaxy model based on optical data, stellar X-ray luminosity functions derived from the full Einstein base, and a model for X-ray absorption derived from hydrogen column densities are used to estimate the contribution to the diffuse soft X-ray background flux from the main-sequence A, F, G, K, and M stars, and RS CVn stars, at various energies ranging from 0.1 to about 5 keV. Previous discrepancies between earlier estimates of the stellar contribution to the diffuse soft X-ray background are resolved; this stellar contribution to the diffuse soft X-ray background is found to be under 3 percent for photon energies less than energy I and J bands (about 0.8-2 keV), at a threshold sensitivity for point source detection about 10 exp -10 ergs/s sq cm. At low latitudes, stellar contribution estimates are less than 3 percent below 0.3 keV, 7-40 percent in the medium-energy bands, and 27-70 percent in the I and J bands. It is shown that while dM stars are the major contributors to the diffuse stellar flux, other stellar types contribute as much as 40 percent of this flux at the higher energies in the passband studied.
Astrophysical Journal v.391, p.667Jun 1992 - Time variability studies with photon-counting imaging detectors. I – A maximum likelihood technique
Abstract
A new method is presented for characterizing the variability properties of a preselected class of celestial objects which are repeatedly observed with the same photon-counting imaging detector. Instrument-specific characteristics, such as detector instrument point response function and mirror system off-axis response, are separated and taken into account in the form of input parameters. All collected source data, both in the form of detections and upper bounds at putative source positions, are retained and fully used in determining the maximum likelihood (ML) estimator of the intrinsic source count rate. For each distinct observation it is determined, at the chosen significance level based on Poisson statistics, whether the observed flux (or upper bound) undergoes a statistically significant variation with respect to the derived ML flux estimator, and variation amplitude, or an upper bound for those cases where significant variations have not been detected is evaluated. It is concluded that, on the basis of presently limited sensitivity data, the variability behavior of the X-ray coronal emission of Pleaides and Hyades stars cannot be distinguished.
Astrophysical Journal v.388, p.595Apr 1992 - The stellar coronal component of the Galaxy. I – The X-COUNT numerical model
Abstract
We present a computational model for the prediction and analysis of the distribution of galactic X-ray stellar coronal source counts, based on models for the distribution of stars and of interstellar hydrogen in the Galaxy and on known properties of stellar X-ray emission in the solar neighborhood. The model can be, and has been used, for comparisons with unbiased X-ray surveys (equivalent to the deep source counts conducted in the optical), for predictions of stellar X-ray observations with new instruments and to estimate the stellar contribution to the X-ray background.
Astronomy and Astrophysics (ISSN 0004-6361), vol. 256, no. 1, p. 86-94.Mar 1992 - Characterization of Long-term X-ray Variability in a Sample of Late-type Stars
Abstract
Not Available
Surface Inhomogeneities on Late-Type Stars, Proceedings of a colloquium held at Armagh Observatory, Northern Ireland, 24-27 July, 1990. Edited by Patrick B. Byrne and Dermott J. Mullan. Lecture Notes in Physics, Vol. 397. Published by Springer-Verlag, Heidelberg, Germany, 1992., p.334n/a 1992
1991
- The diffuse soft X-ray background as seen with the Einstein Observatory
Abstract
A systematic survey of the diffuse soft X-ray background as seen directly with the Einstein Observatory is presented. With the aid of 1633 selected 1 x 1 deg fields of view obtained by the IPC to provide about 5-percent sky coverage, with some bias toward the Galactic plane, the background in the 0.16-3.5 keV spectral region was spatially resolved on this angular scale. Maps of the background are characterized and produced at different energies within the Einstein passband. It is confirmed that the Galactic ridge is not present at energies below 0.33 keV and it is demonstrated that the appearance of the ridge above this energy is not due to hard Galactic sources with a flux above 10 exp -13 ergs/sq cm/s. A southern Galactic region is identified, with l between 80 and 180 deg and b less than -5 deg, where the mean background intensity has the lowest value and is homogeneous within better than 9 percent. The implications of these results for the Galactic structure and for the nature of the extragalactic X-ray background are discussed.
Astrophysical Journal v.380, p.495Oct 1991
1990
- Einstein Observatory coronal temperatures of late-type stars
Abstract
The results are presented of a survey of the coronal temperatures of late-type stars using the Einstein Observatory IPC. The spectral analysis shows that the frequently found one- and two-temperature descriptions are mainly influenced by the SNR of the data and that models using continuous emission measure distributions can provide equally adequate and physically more meaningful and more plausible descriptions. Intrinsic differences in differential emission measure distributions are found for four groups of stars. M dwarfs generally show evidence for high-temperature gas in conjunction with lower-temperature material, while main-sequence stars of types F and G have the high-temperature component either absent or very weak. Very hot coronae without the lower-temperature component appearing in dwarf stars are evident in most of the giant stars studied. RS CVn systems show evidence for extremely hot coronae, sometimes with no accompanying lower-temperature material.
Astrophysical Journal v.365, p.704Dec 1990 - Relationship between optical and X-ray properties of O-type stars surveyed with the Einstein Observatory
Abstract
An X-ray luminosity function is derived for a representative volume-limited sample of O-type stars selected from the catalog of Galactic O stars surveyed with the Einstein Observatory. It was found that, for the stars of this sample which is ten times larger than any previously analyzed, the level of X-ray emission is strongly correlated with bolometric luminosity, confirming previous findings of an Lx-L(bol) relationship (e.g., Harnden et al., 1979; Pallavicini et al., 1981). Correlations between the Lx and the mass loss rate with the wind terminal velocity or with the rotation rate were weak. However, there was a strong correlation with wind momentum flux as well as with the wind kinetic energy flux.
Astrophysical Journal v.361, p.621Oct 1990 - X-ray studies of coeval star samples. III – X-ray emission in the Ursa Major stream
Abstract
Results are reported from a comprehensive survey of X-ray emission from stars known or suspected to be members of the UMa cluster and/or stream. Of the 42 UMa member stars surveyed, 18 were detected as X-ray sources, and spectral analysis was performed for 10 stars with sufficient X-ray counts. Consideration is given to relations between X-ray luminosity, color, and kinematics of the sample stars, and the X-ray spectra of the UMa stars are discussed in the context of the general problem of stellar X-ray temperatures. Also confirmed is the lack of X-ray-emitting A dwarfs among UMa members; among stars of later spectra type there is a rather large dispersion in X-ray luminosity. This dispersion cannot readily be explained by contamination with field star interlopers and appears rather to be a property of the UMa X-ray luminosity distribution function.
Astrophysical Journal, Vol. 351, p. 492-499 (1990)Mar 1990 - X-ray studies of coeval star samples. II – The Pleiades cluster as observed with the Einstein Observatory
Abstract
Coronal X-ray emission of the Pleiades stars is investigated, and maximum likelihood, integral X-ray luminosity functions are computed for Pleiades members in selected color-index ranges. A detailed search is conducted for long-term variability in the X-ray emission of those stars observed more than once. An overall comparison of the survey results with those of previous surveys confirms the ubiquity of X-ray emission in the Pleiades cluster stars and its higher rate of emission with respect to older stars. It is found that the X-ray emission from dA and early dF stars cannot be proven to be dissimilar to that of Hyades and field stars of the same spectral type. The Pleiades cluster members show a real rise of the X-ray luminosity from dA stars to early dF stars. X-ray emission for the young, solarlike Pleiades stars is about two orders of magnitude more intense than for the nearby solarlike stars.
Astrophysical Journal v.348, p.557Jan 1990
1989
- The Einstein X-ray Observatory Catalog of O-type stars
Abstract
A catalog of X-ray count rates for all O-type stars surveyed by the Einstein Observatory is presented. The procedures by which the IPC and High Resolution Imager rates are converted to emitted X-ray luminosities are discussed in detail. The catalog contains 289 stars with 89 detections and 176 upper bounds for the observed count rates. Precise positions for 43 O-type stars are given whose previously known locations were unknown or erroneous. It is argued that most O-type stars are X-ray point sources, but some are possibly embedded in extended nebulae and/or bubbles. The X-ray spectra of O-type stars are discussed briefly, and it is argued that their mean spectrum can reasonably be taken as an 0.5 keV thermal bremsstrahlung model. It is argued that at least 30 percent of all O-type stars are variable in X-rays.
Astrophysical Journal v.341, p.427Jun 1989 - Variability of X-ray emission from OB stars
Abstract
The variability in soft X-ray emission of 12 OB stars is studied. Two different methods of analysis, one more suitable for detecting short-term variations, the other aimed at detecting long time-scale variations, are applied to all stars in the sample. The long-term variability analysis shows that Cyg-OB2 8A Zeta Pup and Delta Ori exhibit significant count rate variations between different data sections. Similar variations are marginally detected in 15 Mon; the count rate variations for the other eight stars are consistent with statistical fluctuations. The light curve of Cyg-OB2 8A suggests the existence of two different emission levels. The short-term variability analysis detects marginal variability in Tau Sco with an effective amplitude of about 30 percent and a time scale of about 50 s. The upper limits to the effective short-term variability amplitude for all other sample stars are in the 10-30 percent range.
Astrophysical Journal v.338, p.296Mar 1989
1988
- The Einstein Observatory survey of stars in the Hyades cluster region
Abstract
The authors report the results of an extensive X-ray investigation of the Hyades region and improve upon previous studies by using refined X-ray source detection algorithms and the complete set of Einstein Observatory IPC exposures covering the Hyades cluster region (a total of 63 1°×1° images). Using a somewhat more extensive and complete compilation of optical candidates, the authors have detected 66 out of 121 Hyades members falling in the combined fields of view. The authors have also computed 3σ upper limits for all the nondetected Hyades members and have derived maximum-likelihood X-ray luminosity functions for the Hyades stars in selected spectral type ranges, using both detections and upper limits.
Astrophysical Journal v.325, p.798Feb 1988 - The stellar composition of X-ray surveys from the Einstein Observatory
Abstract
A new class of X-ray-luminous 'yellow' stellar objects which contributes significantly to the stellar log N-log S distribution, but which cannot be reconciled with normal G and K main-sequence stars. This identification results from a new analysis of the stellar content of three samples of X-ray-selected X-ray sources observed with the Einstein Observatory, namely the 'Medium Sensitivity Survey', the 'High Sensitivity Survey', and the 'Hyades Region Survey'. In this paper, both X-ray and optical properties of the stellar samples in these surveys are reported. The actual stellar content of the surveys is compared with predictions based on current knowledge of stellar X-ray luminosity functions and the stellar composition and spatial distribution in the Galaxy. It is shown that a plausible identification for the excess population of 'yellow' stars is with the active, RS CVn-like binaries.
Astrophysical Journal v.324, p.1010Jan 1988
1987
- Rapid, low-level X-ray variability in active late-type dwarfs
Abstract
The authors report the detection of rapid (a few hundred to >1000 s) quiescent variability, at ≥99% significance, in 24 of 34 Einstein IPC observations of 19 active K and M dwarfs. The IPC light curve and a plot of the variability analysis (using a new, statistically rigorous form of χ2 analysis) are given for each observation. The quiescent fluctuations appear not to fall on an extension of the N(S) curve for stellar X-ray flares. The authors discuss solar analogs and the recent suggestions that low-level flaring heats quiescent X-ray coronae.
Astrophysical Journal Supplement v.65, p.273Oct 1987 - Einstein Observatory survey of X-ray emission from solar-type stars – The late F and G dwarf stars
Abstract
Results of a volume-limited X-ray survey of stars of luminosity classes IV and V in the spectral range F7-G9 observed with the Einstein Observatory are presented. Using survival analysis techniques, the stellar X-ray luminosity function in the 0.15-4.0 keV energy band for both single and multiple sources. It is shown that the difference in X-ray luminosity between these two classes of sources is consistent with the superposition of individual components in multiple-component systems, whose X-ray properties are similar to those of the single-component sources. The X-ray emission of the stars in our sample is well correlated with their chromospheric CA II H-K line emission and with their projected equatorial rotational velocity. Comparison of the X-ray luminosity function constructed for the sample of the dG stars of the local population with the corresponding functions derived elsewhere for the Hyades, the Pleiades, and the Orion Ic open cluster confirms that the level of X-ray emission decreases with stellar age.
Astrophysical Journal v.315, p.687Apr 1987 - Variability analysis in low count rate sources
Abstract
A method, based on the chi-square statistics, is described for detecting pulselike time variability in low count rate sources observed with photon-counting instruments. This method can be used even in the presence of observational gaps, takes full advantage of the filtering effect due to binning with different bin sizes, and takes into account the arbitrariness introduced by the binning phase. The procedure developed to limit the dependence of the results on the binning phase and ensure statistically correct results is described along with the application of the proposed procedure to a model of a variable source. Monte Carlo simulations are used to show how the method can be used to derive the characteristic variability time scales and that the method is more sensitive than the nonparametric Kolmogorov-Smirnov test in detecting variability to a given confidence level.
Astrophysical Journal v.315, p.340Apr 1987
1986
- Multiband far-infrared observations of the NGC 6334 complex
Abstract
Multiband far-infrared (20-155 microns) observations of the large H II region/molecular cloud complex NGC 6334 are presented. The observations were performed simultaneously in four bands with effective wavelengths 21, 42, 71, and 134 microns. Eight far-infrared sources were detected which cover the range from sources with little of no continuum radio emission to well-developed H II regions. Maps of dust temperature and far-infrared optical depth of the complex are presented which reveal large-scale variations in the color temperature and optical depth. Star formation in the complex is discussed and it is concluded that a Rayleigh-Taylor instability generated by the gravitational attraction of the galactic plane can explain the regular separation of the far-IR sources.
Astrophysical Journal v.303, p.629Apr 1986 - The Far-Infrared Spectrum of IRC +10216
Abstract
The infrared source IRC +10216 is a late-type variable carbon star with an extensive envelope of dust and gas caused by mass loss from the central star. Because of the variability of the source and the fact that in the far-IR, calibration uncertainties are generally more than the statistical errors, the authors attempted to measure the spectrum and, if possible, the size, in a single experiment. In this paper, they describe the results of observations from a balloon-borne telescope in the 20 - 160 μm range. The authors measured the fluxes simultaneously in four photometric channels and present the results on the spectrum, and the emissivity of dust grains responsible for the same.
Irish Astronomical Journal, vol. 17(3), p. 313Mar 1986 - X-ray activity
Abstract
The authors review recent observational results related to phenomena of stellar X-ray activity, and extensively discuss the experimental data for the activity from late-type stars in the context of solar coronal modelling analogy. The evidence for a relation between X-ray activity and stellar surface rotational rate, stellar age and the onset of convection along the main sequence are extensively discussed, with some emphasis on the possible changes of X-ray activity manifestation with stellar evolution.
Advances in Space Research, Volume 6, Issue 8, p. 99-107.n/a 1986 - Rapid X-Ray Variability in Einstein Observations of K and M Dwarfs
Abstract
Quiescent variability (at a significant level of ⪆99%) was detected in 15 of 16 late type stars observed by the Einstein IPC.
Cool Stars, Stellar Systems and the Sun, Proceedings of the Fourth Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, Held in Santa Fe, New Mexico, October 16-18, 1985. Lecture Notes in Physics, Vol. 254, edited by Michael Zeilik and David M. Gibson, Springer-Verlag, Berlin Heidelberg New York, 1986., p.219n/a 1986
1985
- Einstein X-ray survey of the Pleiades – The dependence of X-ray emission on stellar age
Abstract
The data obtained with two pointed observations of 1 deg by 1 deg fields of the Pleiades region have been analyzed, and the results are presented. The maximum-likelihood X-ray luminosity functions for the Pleiades G and K stars in the cluster are derived, and it is shown that, for the G stars, the Pleiades X-ray luminosity function is significantly brighter than the corresponding function for Hyades G dwarf stars. This finding indicates a dependence of X-ray luminosity on stellar age, which is confirmed by comparison of the same data with median X-ray luminosities of pre-main sequence and local disk population dwarf G stars. It is suggested that the significantly larger number of bright X-ray sources associated with G stars than with K stars, the lack of detection of M stars, and the relatively rapid rotation of the Pleiades K stars can be explained in terms of the onset of internal differential rotation near the convective envelope-radidative core interface after the spin-up phase during evolution to the main sequence.
Astrophysical Journal, Vol. 292, p. 172-180 (1985)May 1985 - Far-infrared spectrum of IRC + 10216
Abstract
Far-infrared observations of IRC + 10216 in the wavelength range 20-160 microns are presented. The observations were performed simultaneously in four bands with effective wavelengths of 21, 42, 73, and 135 microns, respectively. The scan profile across the source at 21 microns is found to be wider than that for a point source. Interpreting the far-infrared spectrum on the basis of a model in which circumstellar grains are heated by a central source, the radial density dependence is r exp -2, and the emissivity dependence is lambda exp -n, it is found that n = 1-1.2. Time variations of a smaller magnitude at longer wavelengths as compared to the variation at near-infrared wavelengths can also be understood on the basis of this model. Using the mass-loss rate derived from CO observations, the mass absorption coefficient of the grains at 100 microns is found to be not less than 40/sq cm g.
Astrophysical Journal, Vol. 289, p. 630-633 (1985)Feb 1985
1984
- Far-infrared and CO observations of the W33 complex
Abstract
The authors present multi-band far-infrared (20 - 250 μm) and CO observations of the giant H II region W33. At a resolution of ≡1arcmin, they resolve the complex into four distinct far-infrared (far-IR) sources, derive their color temperatures and optical depths, and examine their relationship to the molecular and radio continuum properties. Most of the W33 radio continuum features observed at 3.5 cm have far-IR counterparts. The northern source (W33 A) has a high far-IR luminosity (≡105L_sun;) but is undetectable in the radio continuum. This source is either a normal ZAMS O7 star with an optically thick H II region of density 105 - 106cm-3 or a 30 - 50 M_sun; protostar in its gravitational contraction stage.
Astrophysical Journal, Vol. 283, p. 573-579 (1984)Aug 1984
Not Refereed
2019
- Simultaneous Kepler/K2 and XMM-Newton observations of superflares in the PleiadesAstronomische Nachrichten, Volume 340, Issue 4, pp. 302-307May 2019
2016
- The filter wheel and filters development for the X-IFU instruments onboard Athena
Abstract
Athena is the large mission selected by ESA in 2013 to investigate the science theme "Hot and Energetic Universe" and presently scheduled for launch in 2028. One of the two instruments located at the focus of the 12 m-long Athena telescope is the X-ray Integral Field Unit (X-IFU). This is an array of TES microcalorimeters that will be operated at temperatures of 50 mK in order to perform high resolution spectroscopy with an energy resolution down to 2.5 eV at energies < 7 keV. In order to cope with the large dynamical range of X-ray fluxes spanned by the celestial objects Athena will be observing, the X-IFU will be equipped with a filter wheel. This will allow the user to fine tune the instrument set-up based on the nature of the target, thus optimizing the scientific outcomes of the observation. A few positions of the filter wheel will also be used to host a calibration source and to allow the measurement of the instrument intrinsic background.
Proceedings of the SPIE, Volume 9905, id. 990561 8 pp. (2016).Jul 2016 - Photoevaporation and close encounters: how the environment around Cygnus OB2 affects the evolution of protoplanetary disks
Abstract
In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cygnus OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun, and hosts hundreds of massive stars and thousands of low mass members. In this paper, we analyze the spatial variation of the disk fraction in Cygnus OB2 and we study its correlation with the local values of Far and Extreme ultraviolet radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV field and large stellar density. In particular, the FUV radiation dominates disks dissipation timescales in the proximity (i.e. within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars due to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cygnus OB2, and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cygnus OB2 are potentially hostile to protoplanetary disks, but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.
eprint arXiv:1605.01773May 2016
2015
- Manufacturing and testing a thin glass mirror shell with piezoelectric active control
Abstract
Optics for future X-ray telescopes will be characterized by very large aperture and focal length, and will be made of lightweight materials like glass or silicon in order to keep the total mass within acceptable limits. Optical modules based on thin slumped glass foils are being developed at various institutes, aiming at improving the angular resolution to a few arcsec HEW. Thin mirrors are prone to deform, so they require a careful integration to avoid deformations and even correct forming errors. On the other hand, this offers the opportunity to actively correct the residual deformation: a viable possibility to improve the mirror figure is the application of piezoelectric actuators onto the non-optical side of the mirrors, and several groups are already at work on this approach. The concept we are developing consists of actively integrating thin glass foils with piezoelectric patches, fed by voltages driven by the feedback provided by X-rays. The actuators are commercial components, while the tension signals are carried by a printed circuit obtained by photolithography, and the driving electronic is a multi-channel low power consumption voltage supply developed inhouse. Finally, the shape detection and the consequent voltage signal to be provided to the piezoelectric array are determined in X-rays, in intra-focal setup at the XACT facility at INAF/OAPA. In this work, we describe the manufacturing steps to obtain a first active mirror prototype and the very first test performed in X-rays.
Proceedings of the SPIE, Volume 9603, id. 96031N 9 pp. (2015).Sep 2015 - Solar System and Exoplanets
Abstract
Athena studies of the solar system and exoplanets will enormously advance our understandings on important themes in astrophysics, planetary and space plasma physics such as interactions of space plasmas and magnetic fields, ion and electron acceleration in the planetary magnetosphere, details of the charge exchange process, and physical/chemical evolution of planet atmospheres. We shall review the status of X-ray studies on the solar system and exoplanets, and show the potential of Athena.
Exploring the Hot and Energetic Universe: The first scientific conference dedicated to the Athena X-ray observatory. Proceedings of a conference held 8-10 September, 2015 in Madrid, Spain. Online at http://www.sciops.esa.int/index.php?project=CONF2015&page=ATHENA2015, p.35Sep 2015 - Star Formation and Evolution as seen with Athena
Abstract
Stars over a wide range of masses and evolutionary stages are nowadays known to emit X-rays. This X-ray emission probes the most energetic phenomena occurring in the circumstellar environment of these stars, and provides precious insight on magnetic phenomena or hydrodynamic shocks. Owing to its unprecedented capabilities, Athena will open up an entirely new window on these phenomena. Athena will not only allow us to study many more objects with an unprecedented spectral resolution, but will also pioneer the study of the dynamics of these objects via time-resolved high-resolution spectroscopy. These studies will lead to a deeper understanding of yet poorly understood processes which have profound impact in star formation and evolution, including at earlier times in the Universe. They also are important to understanding feedback processes on Galactic scale.
Exploring the Hot and Energetic Universe: The first scientific conference dedicated to the Athena X-ray observatory. Proceedings of a conference held 8-10 September, 2015 in Madrid, Spain. Online at http://www.sciops.esa.int/index.php?project=CONF2015&page=ATHENA2015, p.36Sep 2015
2014
- Direct sunlight facility for testing and research in HCPV
Abstract
A facility for testing different components for HCPV application has been developed in the framework of "Fotovoltaico ad Alta Efficienza" (FAE) project funded by the Sicilian Regional Authority (PO FESR Sicilia 2007/2013 4.1.1.1). The testing facility is equipped with an heliostat providing a wide solar beam inside the lab, an optical bench for mounting and aligning the HCPV components, electronic equipments to characterize the I-V curves of multijunction cells operated up to 2000 suns, a system to circulate a fluid in the heat sink at controlled temperature and flow-rate, a data logging system with sensors to measure temperatures in several locations and fluid pressures at the inlet and outlet of the heat sink, and a climatic chamber with large test volume to test assembled HCPV modules.
AIP Conference Proceedings, Volume 1616, Issue 1, p.158-161Sep 2014 - Mapping the Star Formation in Orion A/L1641
Abstract
First phases of the process of star formation are characterized by excess in infrared and high X-ray emission. With Spitzer and XMM-Newton we have surveyed the Orion A part relative to the filamentary cloud Lynds 1641 (L1641). Furthermore, an extended spectroscopic survey has been realized to better constraint the cluster membership of stars without IR excess. We find that:
The Labyrinth of Star Formation, Astrophysics and Space Science Proceedings, Volume 36. ISBN 978-3-319-03040-1. Springer International Publishing Switzerland, 2014, p. 291n/a 2014
2013
- Status of the EPIC thin and medium filters on-board XMM-Newton after more than 10 years of operation I: laboratory measurements on back-up filters
Abstract
After more than ten years of operation of the EPIC camera on board the X-ray observatory XMM-Newton, we have reviewed the status of its Thin and Medium filters by performing both laboratory measurements on back-up filters, and analysis of data collected in-flight. We have selected a set of Thin and Medium back-up filters among those still available in the EPIC consortium, and have started a program to investigate their status by different laboratory measurements including: UV/VIS transmission, X-ray transmission, RAMAN IR spectroscopy, X-Ray Photoelectron Spectroscopy, and Atomic Force Microscopy. We report the results of the measurements conducted up to now, and point out some lessons learned for the development and calibration programs of filters for X-ray detectors in future Astronomy missions.
Proceedings of the SPIE, Volume 8859, id. 885914 12 pp. (2013).Sep 2013
2011
- Time-resolved Spectroscopy of DROXO X-ray Sources: Flares and Fe Kα emission
Abstract
We present a systematic search for Fe Kα emission from young stellar objects of the ρ Ophiuchi star forming region observed in the Deep Rho Ophiuchi XMM-Newton Observation.
16th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun. ASP Conference Series, Vol. 448, proceedings of a conference held August 28- September 2, 2010 at the University of Washington, Seattle, Washington. Edited by Christopher M. Johns-Krull, Matthew K. Browning, and Andrew A. West. San Francisco: Astronomical Society of the Pacific, 2012., p.1279Dec 2011
2010
- [Ne II] and X-ray emission from ρ Ophiuchi young stellar objects
Abstract
Circumstellar disks are mostly made of gas. Constraining the spatial and thermal structure of the gas, and its time evolution, is crucial to understand the star- and planet-formation processes. Models predict that the gas is affected by UV and X-ray radiation from the central young stellar object (YSO), but many uncertainties remain, e.g. whether the EUV emission actually reaches the disk or is absorbed by disk winds. The infrared [Ne II] and [Ne III] fine structure lines at 12.81μm and 15.55μm have been theoretically predicted to trace the circumstellar disk gas subject to X-ray heating and ionization.
Highlights of Astronomy, Volume 15, p. 745-745Nov 2010 - VizieR Online Data Catalog: NGC 1893 optical and NIR photometry (Prisinzano+, 2011)
Abstract
We present new optical and NIR photometric data in the VRIJHK and H-α bands for the cluster NGC 1893. The optical photometry was obtained by using images acquired in service mode using two different telescopes: the Device Optimized for the LOw RESolution (DOLORES) mounted on the Telescopio Nazionale Galileo (TNG), used in service mode during three nights in 2007, and the Calar Alto Faint Object Spectrograph (CAFOS), mounted on the 2.2m telescope in Calar Alto German-Spanish Observatory (Spain), during three nights in 2007 and 2008. NIR observations were acquired in service mode at the TNG, using the large field Near Infrared Camera Spectrometer (NICS) with the Js(1.25um), H(1.63um) and K'(2.12um) filters during eight nights in 2007 and 2008. We observed a field around NGC 1893 with a raster of 4x4 pointings, at each pointing we obtained a series of NINT dithered exposures. Each exposure is a repetition of a DIT (Detector Integration Time) times NDIT (number of DIT), to avoid saturation of the background. (4 data files).
VizieR On-line Data Catalog: J/A+A/527/A77. Originally published in: 2011A&A...527A..77POct 2010 - VizieR Online Data Catalog: Stars associated to Eagle Nebula (M16=NGC6611) (Guarcello+ 2010)
Abstract
This catalog contains coordinates and both optical and infrared photometry, plus usefull tags, of the candidate stars associated to the Eagle Nebula (M16), bost disk-less and disk-bearing, selected in Guarcello et al. 2010: "Chronology of star formation and disks evolution in the Eagle Nebula". The optical photometry in BVI bands comes from observations with WFI@ESO (Guarcello et al. 2007, Cat. J/A+A/462/245); JHK photometry have been obtained from 2MASS/PSC (Bonatto et al. 2006A&A...445..567B, Guarcello et al. 2007, Cat. J/A+A/462/245) and UKIDSS/GPS catalogs (Guarcello et al., 2010, in prep.) ; IRAC data are from GLIMPSE public survey (Indebetouw 2007ApJ...666..321I, Guarcello et al., 2009, Cat. J/A+A/496/453); X-ray data from three observations with Chandra/ACIS-I (Linsky et al., 2007, Cat. J/ApJ/654/347, Guarcello et al., 2007, J/A+A/462/245, Guarcello et al. 2010, in prep.). (1 data file).
VizieR On-line Data Catalog: J/A+A/521/A61. Originally published in: 2010A&A...521A..61GAug 2010 - VizieR Online Data Catalog: Blue stars with disk photometry in NGC 6611 (Guarcello+, 2010)
Abstract
This catalog contains coordinates and both optical and infrared photometry, plus usefull tags, of the Blue Stars With Disk (BWE stars) discussed in detail in Guarcello et al. (2010, in prep): "Pre-main sequence stars with disks in the Eagle nebula observed in scattered light". The optical photometry in BVI bands comes from observations with WFI@ESO (Guarcello et al. 2007, Cat. J/A+A/462/245); JHK photometry have been obtained from 2MASS/PSC and UKIDSS/GPS catalogs (Bonatto et al., 2006A&A...445..567B, Guarcello et al., 2007, Cat. J/A+A/462/245 and 2010, in prep); IRAC data are from GLIMPSE public survey (Indebetouw 2007ApJ...666..321I, Guarcello et al., 2009, Cat. J/A+A/496/453); X-ray data from observations with Chandra/ACIS-I (Linsky et al. 2007, Cat. J/ApJ/654/347, Guarcello et al., 2007, Cat. J/A+A/462/245, Guarcello et al., 2010, in prep). BWE stars have been studied by analizing their Spectral Energy Distribution (SED) using the grid of YSO models developed by Robitaille et al. 2006ApJS..167..256R. (1 data file).
VizieR On-line Data Catalog: J/A+A/521/A18. Originally published in: 2010A&A...521A..18GJun 2010
2009
- VizieR Online Data Catalog: Photometry of candidate members of NGC6611 (Guarcello+, 2009)
Abstract
Optical BVI photometry have been obtained from 2.2m WFI@ESO observations; JHK data are from 2MASS Point Sources Catalog; IRAC data are from GLIMPSE catalog. X-ray data have been obtained from a CHANDRA/ACIS-I observation of a field o f 17'x17' centered on the cluster. (1 data file).
VizieR On-line Data Catalog: J/A+A/496/453. Originally published in: 2009A&A...496..453GMar 2009 - The disk/activity connection results from DROXO
Abstract
We present results on the disk/activity connection obtained with the Deep Rho Ophiuchi XMM-Newton Observation (DROXO). We investigate the origin of the [Ne II] 12.81 μm and Fe 6.4 keV lines, two proxies of the effects of X-rays on disks. The analysis of flares confirms the existence of magnetic loops much longer than the stellar radius.
COOL STARS, STELLAR SYSTEMS AND THE SUN: Proceedings of the 15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun. AIP Conference Proceedings, Volume 1094, pp. 317-320 (2009).Feb 2009 - X-ray properties of sources detected in the DROXO survey
Abstract
DROXO is a XMM-Newton large program that explored the ρ Ophiuchi Cloud with a 500 ks observation. We present the main properties of the 111 X-ray sources detected in DROXO. We achieved an average sensitivity of fX~10-15 erg s-1 cm-2 in unabsorbed flux and LX~1028 erg s-1 in luminosity (0.3-10 keV band). Typically the source spectra are highly absorbed (NH~2.1022 cm-2), with a mean plasma temperature of kT~1.9 keV. Saturation of LX/Lbol ratio occurs at masses of M = 0.5-0.7 Msolar and, below these values of masses, the relation between LX and mass found for Orion Nebula YSOs [1] is found valid also for the X-ray luminosities from YSO in ρ Oph.
COOL STARS, STELLAR SYSTEMS AND THE SUN: Proceedings of the 15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun. AIP Conference Proceedings, Volume 1094, pp. 616-619 (2009).Feb 2009 - X-ray observations of IC 348 in light of an updated cluster census
Abstract
IC 348 is an excellent laboratory for studies of low-mass star formation being nearby, compact and rich. A Chandra observation was carried out early in the satellite
COOL STARS, STELLAR SYSTEMS AND THE SUN: Proceedings of the 15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun. AIP Conference Proceedings, Volume 1094, pp. 628-631 (2009).Feb 2009
2008
- X-ray properties of sources detected in the DROXO survey
Abstract
Not Available
"The X-ray Universe 2008" Symposium held in Granada, Spain, 27-30 May, 2008; Published online at http://xmm.esac.esa.int/external/xmm_science/workshops/2008symposium, p.20"Jul 2008
2007
- VizieR Online Data Catalog: BVI photometry in NGC 6611 (Guarcello+, 2007)
Abstract
The optical data have been taken in the BVI bands with the Wield Field Camera (WFI), mounted on the 2.2m telescope of the European Southern Observatory (ESO) in La Silla (Chile). The optical images used in this work were taken on 29 July 2000. (1 data file).
VizieR On-line Data Catalog: J/A+A/462/245. Originally published in: 2007A&A...462..245GSep 2007
2006
- The Palermo XACT facility: a new 35 m long soft x-ray beam-line for the development and calibration of next-generation x-ray observatories
Abstract
The X-ray Astronomy Calibration and Testing (XACT) facility of the Instituto Nazionale di Astrofisica (INAF) at Osservatorio Astronomico di Palermo has recently undergone a major upgrade with the design and construction of a 35 meter long vacuum beam-line operating in the soft X-rays (0.1-20 keV) and the addition of new hardware to meet the requirements for testing and calibration of next generation X-ray missions. We report on the present configuration of the facility and briefly survey the range of its applications.
Space Telescopes and Instrumentation II: Ultraviolet to Gamma Ray. Edited by Turner, Martin J. L.; Hasinger, Günther. Proceedings of the SPIE, Volume 6266, id. 62663F (2006).Jun 2006 - The Deep Rho Ophiuchi XMM-Newton Observation (DROXO)
Abstract
Not Available
Proceedings of the The X-ray Universe 2005 (ESA SP-604). 26-30 September 2005, El Escorial, Madrid, Spain. Editor: A. Wilson, p.111Jan 2006
2005
- High resolution X-ray spectroscopy of pre-main-sequence stars: TWA 5 and PZ Tel
Abstract
We report on the analysis of high resolution X-ray spectra of two pre-main-sequence stars: TWA 5 (observed with XMM-Newton) and PZ Telescopii (observed with Chandra/HETGS). TWA 5 is a classical T Tauri star in the TW Hydrae association while PZ Tel is a rapidly rotating weak-lined T Tauri star in the beta-Pictoris moving group. For both stars we have reconstructed the emission measure distribution and derived the coronal abundances to check for possible patterns of the abundances related to the first ionization potential of the various elements. We have also derived estimates of the plasma density from the analysis of the He-like triplets. We compare the characteristics of our targets with those of other pre-main sequence stars previously analyzed by other authors: TW Hya, HD 98800 and HD 283572. Our findings suggest that X-ray emission from classical T Tauri and weak-lined T Tauri stars is produced in all cases by magnetically-heated coronae, except for TW Hya which has unique plasma temperatures and densities. Moreover we derive that TWA 5 has the same peculiar Ne/Fe abundance ratio as TW Hya.
Proceedings of the 13th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, held 5-9 July, 2004 in Hamburg, Germany. Edited by F. Favata, G.A.J. Hussain, and B. Battrick. ESA SP-560, European Space Agency, 2005., p.399Mar 2005 - X-ray observations of the young open cluster Blanco 1. The XMM-Newton view
Abstract
Not Available
Proceedings of the 13th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, held 5-9 July, 2004 in Hamburg, Germany. Edited by F. Favata, G.A.J. Hussain, and B. Battrick. ESA SP-560, European Space Agency, 2005., p.879Mar 2005
2003
- VizieR Online Data Catalog: UBVRI photometry of NGC 2422 (Prisinzano+, 2003)
Abstract
The data used in this paper are CCD images in the UBVRI pass-bands collected at the 0.9-m telescope of the Cerro Tololo Inter-American Observatory (CTIO) on January 30, 1997. (1 data file).
VizieR On-line Data Catalog: J/A+A/404/927. Originally published in: 2003A&A...404..927PJun 2003 - VizieR Online Data Catalog: X-ray Luminosities in Blanco 1 (Pillitteri+, 2003)
Abstract
We present X-ray Luminosity Distributions (XLDs) of late-type members (dF, dG, dK, dM) of the Blanco 1 cluster, based on ROSAT-HRI data and new astrometric-photometric membership obtained from the GSC-II project. For the first time we present the XLD of dM stars of this cluster. The high metallicity of Blanco 1 allows us to investigate the role of chemical composition on the coronal emission of late-type stars. Comparison between X-ray Luminosity Distributions of Blanco 1 and Pleiades, NGC 2516 and α Per suggests a possible metallicity effect in dM stars. (1 data file).
VizieR On-line Data Catalog: J/A+A/399/919. Originally published in: 2003A&A...399..919PJun 2003 - Monitoring the stability of thin and medium back-up filters of the Newton-XMM EPIC camera
Abstract
We are conducting a measurement program on back-up filters of the XMM-Newton EPIC camera aimed at monitoring possible aging effects during the mission lifetime. One thin and one medium EPIC back-up filters have been stored since 1997 in an environment similar to that one of the flight filters (dry nitrogen box before launch, high vacuum after launch). The transmission of the two filters has been measured periodically in the 1900-10000 angstrom wavelength range where effects of aging would be clearly evident. The preliminary results, after 5 years of monitoring, show that a slight aging effect has occurred on both filters which, however, has no significant impact onto the EPIC calibration for the correct analysis of the X-ray astrophysical observations.
X-Ray and Gamma-Ray Telescopes and Instruments for Astronomy. Edited by Joachim E. Truemper, Harvey D. Tananbaum. Proceedings of the SPIE, Volume 4851, pp. 264-269 (2003).Mar 2003 - The physics of shocks in the proto-stellar environment: Herbig-Haro objects observed with XEUS
Abstract
We present the potential of XEUS for studying the X-ray emission of Herbig-Haro objects, namely of the shocks associated with jets from proto-stellar objects, recently detected for the first time with Chandra and XMM-Newton (cf. Pravdo et al. 2001, Favata et al. 2002a) After discussing the additional evidence of such a phenomena based on the analysis of archival ROSAT data (Favata et al. 2002b) we show how for the first time with XEUS-1 and/or XEUS-2 it will be possible to study in detail the X-ray spectra of Herbig-Haro objects and determine the physical conditions and spatial structure of matter shocked by the protostellar jets, thus determining the influence of jet-induced X-ray emission on circumstellar environments in the star formation phase. Simulations of spectra obtained under different assumptions about the physical conditions of the emitting plasma with current XEUS baseline detectors have been derived and have been presented. We show that even XEUS-1 will make possible to recognize the non-equilibrium condition of the shocked plasma and to recover the physical parameters of the emitting plasma.
XEUS - studying the evolution of the hot universe, held at MPE Garching, March 11-13, 2002. Edited by G. Hasinger, Th. Boller, and A.N. Parmer, MPE Report 281, 2003, p. 331.n/a 2003 - Recent Results and Perspectives of X-ray and Gamma-ray Astronomy
Abstract
Chandra with its exquisite angular resolution and XMM-Newton with its stunning throughput and band-pass are two outstanding X-ray observatories that have given and are giving crucial contributions to our understanding of physical phenomena at work from the nearest to the farthest region of Universe, e.g. in fully convective brown-dwarfs, embedded protostars and young stars in star formation regions, active galaxy nuclei, galaxies and clusters of galaxies, gamma-ray bursts, etc. More recently INTEGRAL has open a new window on the hard X-ray and gamma-ray sky detecting since its early observations a wealth of sources including some GRBs. X-rays and Gamma-rays are telling us about the cosmic cycle of matter from the early Universe till the present day interstellar medium enriched by Supernova explosions. X-rays and Gamma-rays are essential for understanding the violent phenomena occurring in celestial objects under extreme physical conditions that cannot be reached within the present-day solar system. In the next coming years, while Chandra, XMM-Newton and INTEGRAL will continue to provide superb X-ray data, a national mission, AGILE, devoted to the study of the gamma-ray sky, will enter into operation, soon followed by the NASA gamma-ray mission, GLAST, realized with an important hardware contribution by Italian INFN and INAF groups. The next decade will be marked by the launch, currently planned for 2013, of the NASA X-ray astronomy mission Constellation-X, a group of four identical satellites placed in orbit around a Lagrangian point and planned to study the same region of the sky with an angular resolution of 10"-20", obtaining X-ray spectra up to about 80 keV. The Hard X-ray mirror modules are an important foreseen contribution of the Italian community to this large X-ray observatory, togheter with a possible partecipation to the focal-plane detectors based on micro-calorimeters. The European X-ray astronomy community together with the Japanese community is envisaging an ambitious X-ray observatory, XEUS, that with a planned angular resolution of few arcsec and a focal length of 50 meters will be a world-class facility for the astronomy community beyond 2020. This work has been partly supported by ASI and MIUR.
Memorie della Società Astronomica Italiana Supplement, v. 3, p. 322 (2003)n/a 2003 - Mass Segregation in the Open Cluster NGC 2422
Abstract
UBVRI photometry of the open cluster NGC 2422 (age 10^8 yr) down to a limiting magnitude Vsimeq19 is used to study the cluster spatial distribution and the luminosity and mass functions. From the Color-Magnitude Diagram (CMD), we obtained a list of candidate cluster members based on a photometric criterion. Using a comparison field region and an iterative procedure, a correction for contaminating field stars has been inferred in order to obtain the luminosity and the mass functions in the M=0.4 3.5 M_odot range. By analyzing the spatial distribution, we infer that a non-negligible number of cluster stars lie outside our investigated region. We estimate a correction to the mass function of the cluster in order to take into account the complete stellar population. The Present Day Mass Function of NGC 2422 can be represented by a power-law of index α = 3.07 ± 0.08 (rms) in the mass range 0.9 leq M/M_odot leq 2.5 . The index α and the total mass of the cluster are very similar to those of the Pleiades.
EAS Publications Series, Volume 10, 2003, Galactic and Stellar Dynamics, Proceedings of JENAM 2002, held in Porto, Portugal, 3-6 September, 2002. Edited by C. M. Boily, P. Patsis, S. Portegies Zwart, R. Spurzem and C. Theis, pp.163.n/a 2003
2002
- Coronal Variability in the Young Cluster NGC 2516
Abstract
Not Available
Stellar Coronae in the Chandra and XMM-NEWTON Era, ASP Conference Proceedings, Vol. 277. Edited by Fabio Favata and Jeremy J. Drake. ISBN: 1-58381-119-2. San Francisco: Astronomical Society of the Pacific, 2002., p.609Dec 2002 - The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on spectral type for main-sequence stars
Abstract
Not Available
Stellar Coronae in the Chandra and XMM-NEWTON Era, ASP Conference Proceedings, Vol. 277. Edited by Fabio Favata and Jeremy J. Drake. ISBN: 1-58381-119-2. San Francisco: Astronomical Society of the Pacific, 2002., p.557Dec 2002 - X-ray variability in ROSAT-PSPC observations of dF7-dK2 stars
Abstract
Not Available
Stellar Coronae in the Chandra and XMM-NEWTON Era, ASP Conference Proceedings, Vol. 277. Edited by Fabio Favata and Jeremy J. Drake. ISBN: 1-58381-119-2. San Francisco: Astronomical Society of the Pacific, 2002., p.539Dec 2002 - Discovery of X-ray emission from the proto-stellar jet L1551 IRS5 (HH 154)
Abstract
We have for the first time detected X-ray emission associated with a proto-stellar jet, on the jet emanating from L1551 IRS5. The IRS5 proto-star is hidden beyond a very large absorbing column density, making the direct observation of the jet's emission possible. The observed X-ray emission is likely associated with the shock ``working surface'', i.e. the interface between the jet and the circumstellar medium. The X-ray luminosity emanating from the jet is moderate, at LX ~ 3 times 10^29 erg/s, a significant fraction of the luminosity normally associated with the coronal emission from young stars. The spectrum of the X-ray emission is compatible with thermal emission from a hot plasma, with T ~ 0.5 MK, fully compatible with the temperature expected (on the basis of the jet's velocity) for the shock front produced by the jet hitting the circumstellar medium.
Stellar Coronae in the Chandra and XMM-NEWTON Era, ASP Conference Proceedings, Vol. 277. Edited by Fabio Favata and Jeremy J. Drake. ISBN: 1-58381-119-2. San Francisco: Astronomical Society of the Pacific, 2002., p.467Dec 2002 - An XMM-Newton Pencil-Beam Deep Survey in Upper Sco-Cen
Abstract
Not Available
Stellar Coronae in the Chandra and XMM-NEWTON Era, ASP Conference Proceedings, Vol. 277. Edited by Fabio Favata and Jeremy J. Drake. ISBN: 1-58381-119-2. San Francisco: Astronomical Society of the Pacific, 2002., p.389Dec 2002 - Chandra X-ray Observation of the Orion Nebula Cluster
Abstract
Not Available
Stellar Coronae in the Chandra and XMM-NEWTON Era, ASP Conference Proceedings, Vol. 277. Edited by Fabio Favata and Jeremy J. Drake. ISBN: 1-58381-119-2. San Francisco: Astronomical Society of the Pacific, 2002., p.155Dec 2002 - AD Leo observed with Chandra LETG: emission measure distribution density, and element abundances of the coronal plasma
Abstract
Not Available
Stellar Coronae in the Chandra and XMM-NEWTON Era, ASP Conference Proceedings, Vol. 277. Edited by Fabio Favata and Jeremy J. Drake. ISBN: 1-58381-119-2. San Francisco: Astronomical Society of the Pacific, 2002., p.57Dec 2002 - Chandra X-ray Observations of Open Cluster NGC 2516
Abstract
Not Available
Stellar Coronae in the Chandra and XMM-NEWTON Era, ASP Conference Proceedings, Vol. 277. Edited by Fabio Favata and Jeremy J. Drake. ISBN: 1-58381-119-2. San Francisco: Astronomical Society of the Pacific, 2002., p.455Dec 2002 - The Coronal Spectra of Post-T Tauri Stars in the Nearby Tucanae Association
Abstract
We propose to observe the two X-ray brightest single young (10-20 Myr) late-type stars of the recently discovered nearby Tucanae Association. The collected HETG spectra will allow the unique opportunity of characterizing the properties of coronae of late-type active single stars still on the radiative track while contracting toward the main-sequence. This will allow us to investigate the effect of binarity versus youth in determinig the shape of differential emission measure distribution and abundances of stellar coronae in a region of the age-activity-rotation parameter space still essentially unexplored.
Chandra Proposal ID #04200106Sep 2002 - Chandra X-Ray Observations of the Young Cluster NGC 6530
Abstract
We present a preliminary analysis of a Chandra GTO observation of the very young stellar cluster NGC 6530, associated with the H II region M8 (the "Lagoon Nebula") at a distance of about 1800 pc. In a single 60-ksec ACIS-I observation, we detect nearly 900 X-ray point sources, most indeed expected to be cluster members, thereby selecting a rich previously unknown pre-main-sequence population of NGC 6530 and enlarging its stellar census by more than a factor of 10. We are also studying the relationships between X-ray emission and basic stellar properties, finding results similar to those from other star-forming region studies (e.g., the Orion Nebula). These new data also present a rich variety of X-ray spectra and light curves, whose analysis is expected to yield valuable information on the geometrical and thermal structure of the coronae of these stars.
American Physical Society, April Meeting, Jointly Sponsored with the High Energy Astrophysics Division (HEAD) of the American Astronomical Society April 20 - 23, 2002 Albuquerque Convention Center Albuquerque, New Mexico Meeting ID: APR02, abstract #B17.063Apr 2002 - VizieR Online Data Catalog: HRI observations of PMS stars in NGC 2264 (Flaccomio+, 2000)
Abstract
We analyze six ROSAT HRI observations pointed toward the Star Forming Region (SFR) NGC 2264. Three are pointed to the southern star formation core, the other three about 20' to the north. We detect 169 X-ray sources, ~95% of which are likely to be Pre Main Sequence (PMS) stars, significantly enlarging the known population of the SFR in the area covered by the observations. Using published BVRI photometry we place the X-ray sources with well defined optical counterparts on the HRI diagram and estimate their masses and ages. Our comparison of the mass function and age distribution of the X-ray sources with results previously obtained for NGC 2264, demonstrates that deep X-ray observations provide, at least in this case, a very efficient method of selecting SFR members and does not introduce stronger biases than other methods. (2 data files).
VizieR On-line Data Catalog: J/A+A/355/651. Originally published in: 2000A&A...355..651FJan 2002 - VizieR Online Data Catalog: ROSAT medium-sensitivity Galactic plane survey (Morley+, 2001)
Abstract
We have performed a moderately deep soft X-ray (0.1-2keV) survey of the Galactic plane using pointed observations with the ROSAT Position Sensitive Proportional Counter (PSPC). The survey is more than an order of magnitude more sensitive than previous X-ray surveys near the Galactic plane. The data consist of nine fields each of ~10ks exposure, pointed at positions on or very close to the Galactic plane (|b|<0.3{deg}) in the longitude range 180{deg}<~l<~280{deg}. This region has relatively low X-ray absorbing material out to distances of several hundred pc and presents fewer source-confusion problems than at other longitudes. The total sky area surveyed was 2.5deg2 this yielded 93 sources, 89 of which were detected in the hard (0.4-2.0keV) band. Nine sources were detected in both soft (0.1-0.4keV) and hard bands. In the hard band, the survey coverage is 90 per cent for sources brighter than 0.002count/s (~2x10-14erg/cm2/s), but falls steeply below this value, with the weakest sources being ~0.001count/s. The median limiting flux is 0.002count/s. We present the catalogue of X-ray sources and the number-flux relations (logN-logS). Eighteen sources have possible identifications from the SIMBAD data base. We have searched the Tycho-2 (Cat. ) and USNO-A2.0 (Cat. ) catalogues to find all possible optical counterparts brighter than 19th magnitude, and attempt to classify these on the basis of log(FX/Fopt) versus optical colour diagrams and near-infrared photometry from the 2MASS Second Incremental Data Release (Cat. ). Hence, we have found the majority of these sources to be consistent with being late-type main-sequence stars, as previous studies have proposed from incompletely identified surveys. Comparison of the measured number-flux relations with predictions of Galactic (stellar) and extragalactic populations supports the view that the population of young stars in the plane is denser than previously thought. (2 data files).
VizieR On-line Data Catalog: J/MNRAS/326/1161. Originally published in: 2001MNRAS.326.1161MJan 2002
2001
- The Low-Mass Population of NGC2264 and its Coronal Properties.
Abstract
We propose to perform a Chandra ACIS-I 100 ks observation of the star forming region NGC 2264 (in Monoceros), achieving an unprecedent sensitivity of fx ~ 5e-16 erg cm-2 s-1 (0.1-4 kev), or Lx ~ 4e28 erg s-1 (for the commonly assumed distance of 760 pc). This will allow us to detect more than 85% of NGC 2264 low-mass members at least down to 0.1 Msun and a sizable fraction of even lower mass members making possible: 1) to derive a reliable Initial Mass Function (IMF) at the very low-mass end, 2) to investigate the role of magnetic fields in the formation process and resulting IMF, 3) to test the coronal emission level of young brown dwarfs, 4) to study the relation between coronal emission level and PMS evolution during the first 3-10 millon year of stellar lifetime.
Chandra Proposal ID #03200178Sep 2001 - Chandra X-ray Observations of Open Cluster NGC 2516
Abstract
New Chandra X-ray Observatory calibration data have recently been acquired for open cluster NGC 2516 (``the southern Pleiades''). We achieve higher sensitivity than previously possible for the cluster by combining the new data with reprocessed data from existing calibration and GTO observations and adding data from previously obtained but not yet exploited ACIS-S calibration fields. Preliminary results of our new analyses will be presented.
Two Years of Science with Chandra, Abstracts from the Symposium held in Washington, DC, 5-7 September, 2001, meeting abstract.Sep 2001 - The Convective Properties of Late-Type Stars in NGC 2516
Abstract
NGC 2516 is a rich, young open cluster at a similar age to the Pleiades but a factor of two lower metallicity. It has become the focus of attention for studying how metallicity can affect convective processes such as dynamo generated coronal activity and the mixing and depletion of light elements (Li, Be). The cluster has been extensively observed by Chandra and XMM-Newton, which detect many low mass (0.3-1.0M_⊙) X-ray cluster candidates. Our aims here are to: (1) Construct a catalogue of cluster members that is unbiased with respect to X-ray emission and Li abundance, using a combination of photometry, proper motions and radial velocities. (2) Use this catalogue to investigate the dependence of coronal X- ray emission on metallicity. (3) Directly test models of pre-main sequence evolution and surface Li depletion. These predict that low mass stars in NGC 2516 should have suffered an order of magnitude less Li depletion than analogues in the Pleiades. CTIO+Hydra offers a world class facility that will enable us to obtain accurate radial velocities, clearly identifying the cluster members, and simultaneously allowing photospheric Li measurements in stars with much lower masses and deeper convection zones than previously possible.
NOAO Proposal ID #2001A-0223Feb 2001 - Book Review: Stellar clusters and associations : convection, rotation and dynamos / Astronomical Society of the Pacific, 1999
Abstract
Not Available
The Observatory, vol. 121, no. 1160, p. 61 (2001)Feb 2001 - Analysis of Large Stellar Flares: Generalized Evidence for Compact Loops with Sustained Heating (CD-ROM Directory: contribs/favata)
Abstract
Not Available
11th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, ASP Conference Proceedings, Vol. 223. Edited by Ramon J. Garcia Lopez, Rafael Rebolo, and Maria Rosa Zapaterio Osorio. San Francisco: Astronomical Society of the Pacific, ISBN: 1-58381-055-2, p.1133n/a 2001 - Results from the Chandra HRC Survey of the Orion Nebula Cluster Region
Abstract
Not Available
X-ray Astronomy 2000, ASP Conference Proceeding Vol. 234. Edited by Riccardo Giacconi, Salvatore Serio, and Luigi Stella. San Francisco: Astronomical Society of the Pacific. ISBN: 1-58381-071-4, 2001., p.155n/a 2001 - The Chandra HRC Image of the Orion Trapezium Region
Abstract
Not Available
From Darkness to Light: Origin and Evolution of Young Stellar Clusters, ASP Conference Proceedings, Vol. 243. Edited by Thierry Montmerle and Philippe André. San Francisco: Astronomical Society of the Pacific, ISBN: 1-58381-081-1, 2001, p.461n/a 2001 - Prospects for X-ray Observations of SFRs and Young Clusters with Chandra and XMM-Newton
Abstract
Not Available
From Darkness to Light: Origin and Evolution of Young Stellar Clusters, ASP Conference Proceedings, Vol. 243. Edited by Thierry Montmerle and Philippe André. San Francisco: Astronomical Society of the Pacific, ISBN: 1-58381-081-1, 2001, p.605n/a 2001 - Chandra Observations of the Young Open Cluster NGC 2516
Abstract
Not Available
From Darkness to Light: Origin and Evolution of Young Stellar Clusters, ASP Conference Proceedings, Vol. 243. Edited by Thierry Montmerle and Philippe André. San Francisco: Astronomical Society of the Pacific, ISBN: 1-58381-081-1, 2001, p.621n/a 2001 - Results from an XMM-EPIC Observation of Upper Sco-Cen
Abstract
Not Available
Young Stars Near Earth: Progress and Prospects, ASP Conference Series Vol. 244. Edited by Ray Jayawardhana and Thomas Greene. San Francisco: Astronomical Society of the Pacific, ISBN: 1-58381-082-X, 2001., p.171n/a 2001 - X-Ray Surveys and Young Stellar Population: Constraints on Stellar Birthrate in the Solar Neighborhood
Abstract
Not Available
Young Stars Near Earth: Progress and Prospects, ASP Conference Series Vol. 244. Edited by Ray Jayawardhana and Thomas Greene. San Francisco: Astronomical Society of the Pacific, ISBN: 1-58381-082-X, 2001., p.177n/a 2001 - X-Ray Luminosity Functions of the Young Open Cluster Blanco 1
Abstract
We present X-Ray Luminosity Functions (XLFs) of late-type Blanco 1 cluster stars based on ROSAT-HRI data. The high metallicity of this cluster ([Fe/H] = + 0.23) and its age similar to that of Pleiades (100 Myr) make Blanco 1 well suited to explore the effects of chemical composition on stellar activity. Proper motion analysis and new astrometric-photometric membership obtained from GSC-2 catalog data allow us to calculate unbiased XLFs for dF, dG, dK and dM stars. Comparison between XLFs of Blanco 1 and the analogs of Pleiades, NGC 2516 and α Persei shows that the luminosity of the dM stars of Blanco 1 is higher than that of the other clusters, suggesting that metallicity can play a role in determining the coronal activity of dM stars. Other effects as differences in rotational periods distribution may also be relevant in explaining the observed differences.
Astronomische Gesellschaft Abstract Series, Vol. 18., Abstracts of Contributed Talks and Posters presented at the Annual Scientific Meeting of the Astronomische Gesellschaft at the Joint European and National Meeting JENAM 2001 of the European Astronomical Society and the Astronomische Gesellschaft at Munich, September 10-15, 2001, abstract #MS 07 14.n/a 2001
2000
- The Chandra HRC Image of the Orion Trapezium Region
Abstract
We present results of a 63 ksec Chandra HRC-I observation of the Orion Trapezium region. We have detected about 700 distinct sources most of which associated with known Orion Nebula Cluster members. The superb angular resolution of the Chandra mirrors frees our analysis from the source confusion problems that have plagued previous investigations of the X-ray emission of this fascinating region and allow us to explore the relations among X-ray emission levels and other stellar parameters in a large sample of PMS stars. This work is funded in part by NASA contract NAS8-39073 and NASA grant NAS5-4967
American Astronomical Society, HEAD Meeting #5, id.42.13; Bulletin of the American Astronomical Society, Vol. 32, p.1257Oct 2000 - CHANDRA Observations of the Open Cluster NGC 2516
Abstract
Not Available
Star formation from the small to the large scale. ESLAB symposium (33 : 1999 : Noordwijk, The Netherlands). Edited by F. Favata, A. Kaas, and A. Wilson. Proceedings of the 33rd ESLAB symposium on star formation from the small to the large scale, ESTEC, Noordwijk, The Netherlands, 2-5 November 1999 Noordwijk, The Netherlands: European Space Agency (ESA), 2000. ESA SP 445., p.559Jun 2000 - Stellar Clusters and Associations: Convection, Rotation, and Dynamos
Abstract
Not Available
Stellar Clusters and Associations: Convection, Rotation, and Dynamos. Proceedings from ASP Conference, Vol. 198. Edited by R. Pallavicini, G. Micela, and S. Sciortino ISBN: 1-58381-025-0 (2000)n/a 2000 - Determination of convective turnover times in young stars
Abstract
Not Available
Stellar Clusters and Associations: Convection, Rotation, and Dynamos. Proceedings from ASP Conference, Vol. 198. Edited by R. Pallavicini, G. Micela, and S. Sciortino ISBN: 1-58381-025-0 (2000), p.71n/a 2000 - An HRI survey of NGC 2264: properties of X-ray emission and IMF determination
Abstract
Not Available
Stellar Clusters and Associations: Convection, Rotation, and Dynamos. Proceedings from ASP Conference, Vol. 198. Edited by R. Pallavicini, G. Micela, and S. Sciortino ISBN: 1-58381-025-0 (2000), p.187n/a 2000 - Proper motions of X-ray detected stars in PSPC ROSAT pointed fields of Chamaeleon Region
Abstract
Not Available
Stellar Clusters and Associations: Convection, Rotation, and Dynamos. Proceedings from ASP Conference, Vol. 198. Edited by R. Pallavicini, G. Micela, and S. Sciortino ISBN: 1-58381-025-0 (2000), p.207n/a 2000 - Study of the near-IR spectra of Weak-line T Tauri stars
Abstract
Not Available
Stellar Clusters and Associations: Convection, Rotation, and Dynamos. Proceedings from ASP Conference, Vol. 198. Edited by R. Pallavicini, G. Micela, and S. Sciortino ISBN: 1-58381-025-0 (2000), p.499n/a 2000 - Galactic X-ray surveys: the role of small missions
Abstract
Not Available
Stellar Clusters and Associations: Convection, Rotation, and Dynamos. Proceedings from ASP Conference, Vol. 198. Edited by R. Pallavicini, G. Micela, and S. Sciortino ISBN: 1-58381-025-0 (2000), p.545n/a 2000 - X-ray observations and stellar physics.
Abstract
Not Available
Memorie della Societa Astronomica Italiana, Vol. 71, p. 935-944 (2000)n/a 2000 - Tracing the star formation history in Taurus-Auriga
Abstract
Not Available
SARG at TNG: Prespectives for the Year 2000, Proceedings of the workshop held in Padova at the Padova Astronomical Observatory, December 20-21, 1999. Edited by R. G. Gratton and R. U. Claudi. Published by the Astronomical Observatory of Padova, Vicolo delln/a 2000 - The origin of Lithium in normal late K dwarfs
Abstract
Not Available
SARG at TNG: Prespectives for the Year 2000, Proceedings of the workshop held in Padova at the Padova Astronomical Observatory, December 20-21, 1999. Edited by R. G. Gratton and R. U. Claudi. Published by the Astronomical Observatory of Padova, Vicolo delln/a 2000 - The Coronae of AD Leo and EV Lac
Abstract
Not Available
Very Low-mass Stars and Brown Dwarfs, Edited by R. Rebolo and M. R. Zapatero-Osorio. Published by the Cambridge University Press, UK, 2000., p.248n/a 2000 - The XACT facility at OAPA : ongoing projects and future developments
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 71, p.1127n/a 2000
1999
- Chandra Observations of Open Cluster NGC 2516. I. The Data
Abstract
The Chandra X-ray Observatory, launched 1999 July 23, spent its first two months performing On-orbit Activation and Calibration (OAC) activities. This phase of the mission acquired about 40 Ks of data from the young cluster NGC 2516 as part of the boresight and plate-scale measurements of the various instruments. These observations will augment a planned (deeper) GTO observation of this cluster. Here we present the data reduction and analysis techniques needed to make scientific use of these early observations. As many as 100 X-ray detections were found in data returned by the ACIS-I, ACIS-S, HRC-I and HRC-S observations. We assess the absolute position accuracy achieved by comparison with ROSAT detections and optical locations of the same cluster members, and we also discuss the methods employed to obtain reliable photometry of the Chandra sources as observed in all four of Chandra's imaging detectors. This work was funded in part by NASA contract NAS8-39073 and NASA grant NAS5-4967.
American Astronomical Society, 195th AAS Meeting, id.47.04; Bulletin of the American Astronomical Society, Vol. 31, p.1441Dec 1999 - Chandra Observations of Open Cluster NGC 2516. II. Interpretation
Abstract
We present the results of an analysis of Chandra X-ray Observatory data for the open cluster NGC 2516, sometimes referred to as ``the southern Pleiades''. This young, very rich southern open cluster is generally believed to have a metallicity significantly below solar. The study of coronal emission properties of NGC 2516 members allows us to probe the effects of metallicity on the efficiency of dynamo action. Dynamo efficiency is expected to depend on convection zone properties and be strongly affected by metal abundances. Our analysis of both HRC and ACIS images has yielded about 100 X-ray detections. We present the identifications of new X-ray sources with photometric cluster members and compare our new Chandra results with those of ROSAT. We also discuss the X-ray properties of late-type members in comparison with those of corresponding stellar types in the more metal rich approximately coeval Pleiades Cluster. This work was funded in part by NASA contract NAS8-39073 and NASA grant NAS5-4967.
American Astronomical Society, 195th AAS Meeting, id.47.05; Bulletin of the American Astronomical Society, Vol. 31, p.1442Dec 1999
1998
- VizieR Online Data Catalog: ROSAT HRI observations of the Pleiades (Micela+ 1999)
Abstract
In a deep X-ray survey of the Pleiades open cluster, we use the ROSAT High Resolution Imager to explore a region of the cluster formerly surveyed with the PSPC. These new observations substantially improve upon both the sensitivity and the spatial resolution for this region of the Pleiades, allowing us to detect 18 cluster members not detected before and 16 members not included in the catalogs used in previous surveys. The high sensitivity of the present observations permits us to obtain more stringent upper limits for 72 additional members and also provides sufficient numbers of stars to enable us to explore the dependence of Lx on stellar rotation for the slow rotators of the Pleiades. Using the new high sensitivity X-ray observations and the recent rotational measurements we discuss the activity-rotation relationship in the Pleiades solar type stars. We also present new photometric observations of optical counterparts of a number of X-ray sources detected in previous surveys but not yet identified. (2 data files).
VizieR On-line Data Catalog: J/A+A/341/751. Originally published in: 1999A&A...341..751MOct 1998 - EV Lacertae
Abstract
IAUC 6987 available at Central Bureau for Astronomical Telegrams.
IAU Circ., No. 6987, #3 (1998). Edited by Green, D. W. E.Aug 1998 - VizieR Online Data Catalog: ROSAT Observations of the Pleiades (Micela+ 1996)
Abstract
The nucleus of the composite catalog used in this study is the catalog compiled from the published literature for the Einstein investigations of the Pleiades (Micela et al. 1990). This list has been extended by the results of recent surveys to a completeness limit of visual magnitude about 18. (3 data files).
VizieR On-line Data Catalog: J/ApJS/102/75. Originally published in: 1996ApJS..102...75MJul 1998 - Simulating the Source Parameter Recovery Capability from Coronal X-ray Spectra: the SAX/LECS and the ASCA/SIS Cases
Abstract
We have studied the performance of global chi^2 fitting applied to low resolution X-ray spectroscopy, focusing on the retrieval of source parameters, with emphasis on the coronal metallicity. The study has been conducted by fitting large numbers of simulated spectra with known characteristics, and studying the distribution of the best-fit parameters, using the responses of the LECS detector onboard the SAX satellite and of the SIS detector onboard the ASCA satellite. The simulations have been done under the hypothesis that the intrinsic source spectrum can be described by two discrete temperatures. The performance of the fitting process has been studied both in the case in which just the global metallicity is left free to vary and in the case in which the abundance of each element is independently left free to vary in the fit process.
ASP Conf. Ser. 154, The Tenth Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, Edited by R. A. Donahue and J. A. Bookbinder, p.1027n/a 1998 - Evolution of X-ray Activity of 1-3 M_odot Late-Type Stars in Early Post-Main-Sequence Phases
Abstract
We present preliminary results on the variation of coronal X-ray emission during early post-main-sequence phases, for a sample of 83 late-type stars (D < 100 pc), with estimated masses in the range 1 - 3 M_odot using Hipparcos parallaxes and recent evolutionary tracks (Schaller et al. 1992). These stars were observed with the ROSAT/PSPC; source detection and evaluation of X-ray fluxes (or upper limits) were achieved with the wavelet transform algorithm of Damiani et al. (1997a,b). The aim of this work is to study the evolutionary history of X-ray activity in early post-main-sequence phases for stars with masses 1.5<M/M_odot<3.0 (inactive A-type stars on the main-sequence) with respect to lower mass solar-type stars. A trend of increasing X-ray luminosity with age for the higher mass stars is suggested by our ROSAT sample, and a similar evidence comes from two other independent samples: a sample of 24 stars observed with the {\it Einstein} satellite (Maggio et al. 1993), and a sample of 23 evolved stars extracted from the RIASS ``coronathon" (Ayres et al. 1995). The results of the present study may help to test the expectation that magnetic dynamo activity depends on the evolutionary phase through several possible ingredients, and in particular the rotation history of coronal X-ray sources during the main-sequence lifetime, and mass-dependent changes of the stellar internal structure.
ASP Conf. Ser. 154, The Tenth Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, Edited by R. A. Donahue and J. A. Bookbinder, p.1146n/a 1998 - X-Ray Source Detection in EPIC Images using Wavelet Transforms
Abstract
Not Available
"Science with XMM, Conference held at ESTEC, Noordwijk, The Netherlands, Sep. 30- Oct. 2, 1998. Published online at http://xmm.esac.esa.int/external/xmm_science/workshops/1st_workshop, article #29"n/a 1998 - The Nature of the Shell Type SNR X-Ray Emission
Abstract
Not Available
"Science with XMM, Conference held at ESTEC, Noordwijk, The Netherlands, Sep. 30- Oct. 2, 1998. Published online at http://xmm.esac.esa.int/external/xmm_science/workshops/1st_workshop, article #80"n/a 1998
1997
- Identifications of Selected Einstein Slew Survey Sources with the ROSAT HRI
Abstract
We present the analysis of a series of pointed ROSAT High Resolution Imager (HRI) observations of X-ray sources detected in the Einstein Slew Survey. Of the 16 slew sources considered here, 10 have been positively identified with single objects -- six with cool-type stars (spectral types F-K), one each with a variable star, a B2V, and a dMe star, and one with a BL Lac object. The HRI fluxes are consistent with those measured by the ROSAT All Sky Survey, for their 12 common detections, and improved positions are obtained for these sources. We calculate values of fX/fV and LX for these stars where possible, and find these measurements to be consistent with those expected for their spectral type.
American Astronomical Society, 191st AAS Meeting, id.03.15; Bulletin of the American Astronomical Society, Vol. 29, p.1209Dec 1997 - The Evolutionary Status of High-Lithium, High-Activity Cool Dwarfs
Abstract
We have used the newly available Hipparcos precise parallaxes to study the evolutionary status of a number of high-lithium, high-activity late-type dwarfs found in X-ray based surveys, the Einstein Extended Medium Sensitivity Survey and the Einstein Slew Survey. The sample stars are placed on evolutionary tracks thus estimating their age.
Proceedings of the ESA Symposium `Hipparcos - VeniceAug 1997 - HIPPARCOS Distances of X-ray Selected Stars: Implications on their Nature
Abstract
We present the parallaxes, measured by Hipparcos, for a sample of X-ray selected stars. The stars belong to the stellar sample of the Einstein Extended Medium Sensitivity Survey. They are all at galactic latitude |b| > 20 deg, and are generally far away from known star forming regions. Several of these stars show lithium abundance and activity level typical of very young stars with ages comparable to that of the Pleiades. We show that the majority of our sample stars are on the main sequence, with ~= 20 per cent being giants. We do not find a significant presence of pre-main sequence stars in our sample, notwithstanding the fact that some of the stars have a considerable lithium abundance, showing that they are most likely young and active main-sequence objects.
Proceedings of the ESA Symposium `Hipparcos - VeniceAug 1997 - Cool stars in clusters and associations: magnetic activity and age indicators. Proceedings. Workshop, Palermo (Italy), 20 – 23 May 1997.
Abstract
The following topics were dealt with: evolutionary models and age determinations; membership determination; proper motion surveys and CCD photometry; rotation and angular momentum evolution; lithium abundance in clusters and associations; chromospheric activity in cluster stars; X-ray observations of open clusters; star formation and young stellar objects; X-ray surveys of star forming regions; solar-type coronal loop modelling of X-ray flares.
Memorie della Societa Astronomica Italiana, Vol. 68, N. 4, p. 783 - 1107n/a 1997 - The evolutionary status of high lithium, high activity cool dwarfs
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 68, p.929n/a 1997 - X-ray observations of the young open cluster ζ Sculptoris
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 68, p.989n/a 1997 - X-Ray emitting PMS stars in Upper Sco-Cen: the X-ray faint population
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 68, p.1069n/a 1997 - X-Ray and optical observations of the star forming region NGC 2264
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 68, p.1073n/a 1997 - A ROSAT deep survey of the galactic plane 180° < l < 280°
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 68, p.1089n/a 1997 - Statistical Properties of Wavelet Transforms Applied to X-Ray Source Detection
Abstract
Not Available
Statistical Challenges in Modern Astronomy II, Proceedings of the conference held 2-5 June, 1996 at Pennsylvania State University. Edited by G.J. Babu and E.D. Feigelson. Berlin: Springer-Verlag, 1997., p.417n/a 1997 - The EUV/X-ray Astronomy Calibration and Testing Facility at the Osservatorio Astronomico di Palermo
Abstract
The X ray Astronomy Calibration and Testing facility (XACT) set-up at the Osservatorio Astronomico di Palermo "G.S. Vaiana" for the development and calibration of instrumentation for Extreme Ultraviolet (EUV)/X ray astronomy, includes a 14 (expandable to 17) meter long stainless-steel vacuum beam line, a 1 meter diameter cylindrical test chamber opening to a class 1000 clean-room, and X ray and EUV sources, detectors and monochromators covering the energy range approximately 0.005-10 keV (wavelength range approximately 1-3000 A). The XACT facility is presently employed in the calibration of the UV/Ion shields of the High Resolution Camera (HRC) on AXAF, in the development of the filters for the EPIC camera on XMM and in the calibration of the JET-X detector. Additions to the basic equipment are planned for the development of cryogenic detectors. A description of the facility can be found at the WWW site: http://www.astropa.unipa.it/XACT/XACT.html
The Next Generation of X-Ray Observatories, p. 153Jan 1997
1996
- Calibration of the AXAF-HRC UV/ion shields at Osservatorio Astronomico DI Palermo G.S. Vaiana: I -instrumental setup
Abstract
We describe the equipment and the experimental set-up at the XACT facility -- Osservatorio Astronomico di Palermo G.S. Vaiana, employed in the calibration of the AXAF-HRC UV/ion shield. The facility has been expanded to improve its performance and to extend its bandpass to UV wavelengths. It now covers the 2550 - 1 angstrom wavelength range, is equipped with an advanced micropositioning system, has imaging capabilities in the wavelength range 1200 - 1 angstrom. The test chamber now opens to a class 1000 clean room which, along with the oil-free pumping system, makes cleanliness one of the characteristics of the facility
Proc. SPIE Vol. 2808, p. 144-155, EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy VII, Oswald H. Siegmund; Mark A. Gummin; Eds.Oct 1996 - EPIC system onboard the ESA XMM
Abstract
The European photon imaging camera (EPIC) is one of the two main instruments onboard the ESA X-Ray Cornerstone Mission XMM. It is devoted to performing imaging and spectroscopy of the x-ray sky in the domain 0.1 10 keV with a peak sensitivity in 10(superscript 5) seconds of 2 multiplied by 10(superscript -15) erg/cm(superscript -2). The x-ray instrumentation is complemented by a radiation monitor which will measure the particle background. The spectral resolution is approximately 140 eV at 6.4 keV and 60 eV at 1 keV. The instrumentation consists of three separate focal plane cameras at the focus of the three XMM telescopes, containing CCDs passively cooled to typically minus 100 degrees via radiators pointing toward the anti-Sun direction. The two cameras with the field of view partially occulted by the RGS grating boxes will have MOS technology CCDs while the third camera, with full field of view, will be based on p-n technology. The CCDs in the focal plane of the cameras will cover the entire 30 foot by 30 foot field of view of the telescope while the pixel size (40 by 40 (mu) for the MOS camera and 150 multiplied by 150 (mu) for the p-n) will be adequate to sample the approximately 20' PSF of the mirrors. In order to cope with a wide range of sky background and source luminosity in the visible/UV band, a filter wheel with six positions has been implemented in each camera. The six positions correspond to: open position, closed position, one thin filter (1600 angstrom of plastic support and 400 angstrom of Al), one medium filter (1600 angstrom of plastic support and 800 angstrom of Al) and one thick filter (approximately 3000 angstrom of plastic support, approximately 1000 angstrom of Al and 300 Angstrom of Sn). The final position will be a redundant filter of type still to be decided. A set of radioactive sources in each camera will allow the calibration of the CCDs in any of the operating modes and with any filter wheel position. Vacuum doors and valves operated will allow the operation of other camera heads on the ground, in a vacuum chamber and/or in a controlled atmosphere, and will protect the CCDs from contamination until the spacecraft is safely in orbit. The MOS camera will have 7 CCDs, each of 600 by 600 pixels arranged in a hexagonal pattern with one central and six peripheral. The p-n camera head will have 12 CCDs, each with 200 multiplied by 64 pixels, in a rectangular arrangement, 4 quadrants of 3 CCDs each. The radiation monitor is based on two separate detectors to monitor the low (electrons greater than 30 keV) and the high (electrons greater than 200 keV and protons greater than 10 MeV) energy particles impinging on the telescope along its orbit.
Proc. SPIE Vol. 2808, p. 402-413, EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy VII, Oswald H. Siegmund; Mark A. Gummin; Eds.Oct 1996 - X-ray variability of T Tauri stars in the ρ Oph region.
Abstract
The authors report on ROSAT HRI observations of two T Tau stars, SR 9 and SR 12, in the ρ Oph star formation region. Each star was observed six times at regular rotational phase intervals over its rotation period, known from optical photometry. Significant X-ray variability has been detected for both stars in the HRI data.
International Conference on X-ray Astronomy and Astrophysics: Röntgenstrahlung from the Universe, p. 27 - 28Feb 1996 - HRI observations of the Pleiades.
Abstract
Preliminary analysis of data from the first four ROSAT HRI pointings has provided many new faint Pleiades detections. Completion of the high-resolution survey of the most source-confused regions of this open cluster will permit the construction of proper X-ray luminosity functions and yield a definitive assessment of the coronal emission of Pleiades members.
International Conference on X-ray Astronomy and Astrophysics: Röntgenstrahlung from the Universe, p. 43 - 44Feb 1996 - On the study of NEI regime in SNR
Abstract
The authors developed and tested a non-equilibrium of ionization (NEI) emission model to study the X-ray emission of supernova remnants (SNRs) with a parametric fitting approach. They present results of ROSAT PSPC spectral fitting simulations with their emission model. They have found that, the confidence ranges of the plasma temperature tend to be larger with respect to fits with equilibrium of ionization models, the χ2 function is characterized by local minima in the parameter space, and there is a strong dependence of the fit results on the initial value of the ionization time used in the minimization algorithm. They explain these findings in terms of the particular nature of the model. It is concluded that, in typical cases of young and middle-aged SNR with a moderate ISM absorption, the local minima can be avoided by the χ2 fitting spectra with 5000 counts or more. Finally, the authors briefly discuss how this limit affects the feasibility of spatially resolved spectral analysis of SNRs observed with the PSPC.
International Conference on X-ray Astronomy and Astrophysics: Röntgenstrahlung from the Universe, p. 235 - 236Feb 1996 - Early-type galaxies: automated reduction and analysis of ROSAT PSPC data.
Abstract
The authors present initial results of early-type galaxies that will be part of a galaxy catalogue (XGALCAT) to be derived from the complete ROSAT archive. The archive data are reduced and analysed via an automatic pipeline that has been developed by the authors.
International Conference on X-ray Astronomy and Astrophysics: Röntgenstrahlung from the Universe, p. 385 - 386Feb 1996 - Time screening optimization algorithm for ROSAT PSPC/HRI observations.
Abstract
ROSAT PSPC/HRI observations are characterized by intervals with significant background count-rate enhancements mainly due to particles or solar scattered radiation. The authors have developed an algorithm for time screening PSPC/HRI observations in order to maximize the signal to noise (SNR) ratio of faint point-like sources. The algorithm parameters have been optimized with tests on real and simulated data. In the case of a crowded PSPC field pointed to the Pleiades open cluster, a typical SNR gain of ≡20% with respect to the unscreened case is obtained for faint sources with a cut of 6% of the total exposure time. This algorithm although presently tailored for the ROSAT PSPC/HRI observations can be easily generalized to the analysis of other X-ray detectors' data.
International Conference on X-ray Astronomy and Astrophysics: Röntgenstrahlung from the Universe, p. 673 - 674Feb 1996 - A detection method for X-ray images based on wavelet transforms: the case of the ROSAT PSPC.
Abstract
The authors have developed a method based on wavelet transforms (WT) to detect efficiently sources in PSPC X-ray images. The multiscale approach typical of WT can be used to detect sources with a large range of sizes, and to estimate their size and count rate. Significance thresholds for candidate detections (found as local WT maxima) have been derived from a detailed study of the probability distribution of the WT of a locally uniform background. The use of the exposure map allows good detection efficiency to be retained even near PSPC ribs and edges. The algorithm may also be used to get upper limits to the count rate of undetected objects. Simulations of realistic PSPC images containing either pure background or background+sources were used to test the overall algorithm performances, and to assess the frequency of spurious detections (vs. detection threshold) and the algorithm sensitivity. Actual PSPC images of galaxies and star clusters show the algorithm to have good performance even in cases of extended sources and crowded fields.
International Conference on X-ray Astronomy and Astrophysics: Röntgenstrahlung from the Universe, p. 679 - 680Feb 1996 - A Source Detection Method for ROSAT/PSPC X-Ray Images based on Wavelet Transforms
Abstract
We have developed a method based on wavelet transforms (WT) to detect sources in ROSAT PSPC X-ray images. The WT, evaluated at a set of scale parameters a, detects efficiently both point-like and extended sources. This method is especially suitable when the PSF width changes strongly across the field of view as in PSPC images. The reliability of the algorithm is ensured even in the presence of strong background gradients (detector ribs) through the use of exposure maps. The WT, computed at a scale a, is mostly sensitive to structures of size d ~ a, estimating source extent, while the height of a WT maximum gives a measure of source counts. The WT probability distribution of the background has been computed through extensive simulations, in the low-background limit and analytically in the high-background limit, to set significance thresholds for candidate sources (i.e., local WT maxima). Realistic simulations of PSPC images, containing both pure background and background plus sources, assess the frequency of spurious detections as a function of detection threshold, and the algorithm sensitivity, respectively. The algorithm may also consistently yield upper limits to the X-ray count rate of undetected objects.
Astronomical Data Analysis Software and Systems V, A.S.P. Conference Series, Vol. 101, 1996, George H. Jacoby and Jeannette Barnes, eds., p. 143.n/a 1996 - A Reduction and Analysis Pipeline for ROSAT PSPC Data
Abstract
We describe a pipeline to reduce and analyze ROSAT Position-Sensitive Proportional Counter (PSPC) archive data. The pipeline is based on an IRAF Command Language (CL) script, that primarily consists of modules comprising IRAF and PROS tasks. In addition, DBMS routines are called from within the script, as well as new data screening and detection software specially developed for PSPC data. The pipeline output includes reduced images, contour maps, surface brightness profiles (for extended sources), spectra, color and hardness ratios as well as data files ready for ingestion into a SYBASE/INGRES Database. We will show examples of pipeline output, sample SYBASE extraction procedures and resulting output, discuss the time requirements for pipeline operation, and summarize the state of pipeline operation on the available archive data.
Astronomical Data Analysis Software and Systems V, A.S.P. Conference Series, Vol. 101, 1996, George H. Jacoby and Jeannette Barnes, eds., p. 179.n/a 1996 - ROSAT PSPC/HRI Observations of the Young Open Cluster NGC 2422
Abstract
Not Available
Cool stars; stellar systems; and the sun : 9 : Astronomical Society of the Pacific Conference Series, volume 109; Proceedings of the 9th Cambridge workshop; held 3-6 October 1995 in Florence; Italy; San Francisco: Astronomical Society of the Pacific (ASP); |c1996; edited by Roberto Pallavicini and Andrea K. Dupree, p.355n/a 1996 - ROSAT HRI survey of the Pleiades
Abstract
Not Available
Cool stars; stellar systems; and the sun : 9 : Astronomical Society of the Pacific Conference Series, volume 109; Proceedings of the 9th Cambridge workshop; held 3-6 October 1995 in Florence; Italy; San Francisco: Astronomical Society of the Pacific (ASP); |c1996; edited by Roberto Pallavicini and Andrea K. Dupree, p.359n/a 1996 - Constraining the low-mass stellar mass-function
Abstract
Not Available
Cool stars; stellar systems; and the sun : 9 : Astronomical Society of the Pacific Conference Series, volume 109; Proceedings of the 9th Cambridge workshop; held 3-6 October 1995 in Florence; Italy; San Francisco: Astronomical Society of the Pacific (ASP); |c1996; edited by Roberto Pallavicini and Andrea K. Dupree, p.365n/a 1996 - X-Ray variability of T Tauri stars in the rho OPH region
Abstract
Not Available
Cool stars; stellar systems; and the sun : 9 : Astronomical Society of the Pacific Conference Series, volume 109; Proceedings of the 9th Cambridge workshop; held 3-6 October 1995 in Florence; Italy; San Francisco: Astronomical Society of the Pacific (ASP); |c1996; edited by Roberto Pallavicini and Andrea K. Dupree, p.423n/a 1996 - An Analysis of X-ray Flares in Pleiades Stars
Abstract
Not Available
Magnetodynamic phenomena in the solar atmosphere. Prototypes of stellar magnetic activity Proceedings of the 153rd colloquium of the International Astronomical Union held in Makuhari; near Tokyo; Japan; May 22-27; 1995; Dordrecht: Kluwer Academic Publishers; |c1996; edited by Yutaka Uchida, Takeo Kosugi, and Hugh S. Hudson, p.277n/a 1996 - Coronal Properties of Nearby Old Disk and Halo dM Stars
Abstract
Not Available
Magnetodynamic phenomena in the solar atmosphere. Prototypes of stellar magnetic activity Proceedings of the 153rd colloquium of the International Astronomical Union held in Makuhari; near Tokyo; Japan; May 22-27; 1995; Dordrecht: Kluwer Academic Publishers; |c1996; edited by Yutaka Uchida, Takeo Kosugi, and Hugh S. Hudson, p.479n/a 1996
1995
- Lithium in Nearby Main-Sequence Solar-Type Stars
Abstract
Not Available
The Light Element Abundances, Proceedings of an ESO/EIPC Workshop Held in Marciana Marina, Isola dn/a 1995
1994
- X-ray Astronomy Calibration and Testing Facility (XACT) at Osservatorio Astronomico DI Palermo G.S. Vaiana
Abstract
We describe the X-ray Astronomy Calibration and Testing Facility of the Osservatorio Astronomico di Palermo G.S. Vaiana. The facility, including a 16 meter vacuum beam line, a 1 m diameter test chamber, a X-ray source system, X-ray detectors, an advanced data acquisition and control system and other minor tools and accessories, features characteristics of vacuum cleanliness and versatility almost unique among facilities of this size. It started operating in June 1993 and will soon be completed with further equipments to improve energy resolution and to extend its capabilities to the UV band up to wavelengths of the order of 3100 A. Possible applications are test and calibration of filters, development and calibration of detectors, reflectivity measurements, testing of small X-ray telescopes. Presently it is employed for the calibration of UV-Ion shields of the High Resolution Camera of AXAF-I.
Proc. SPIE Vol. 2280, p. 206-213, EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy V, Oswald H. Siegmund; John V. Vallerga; Eds.Sep 1994 - Diffuse X-Ray Shadow in the Pleiades
Abstract
We report the results of our analysis of the diffuse X-ray background present in the ROSAT PSPC observations of the core of the Pleiades Cluster. We detect the existence of an X-ray shadow in the 0.2-0.41 keV passband coinciding with IRAS emission. However, the depth of this shadow is significantly less than expected, casting doubts on the generality of the X-ray shadowing observed previously with ROSAT.
American Astronomical Society, 184th AAS Meeting, id.29.13; Bulletin of the American Astronomical Society, Vol. 26, p.903May 1994 - 15-30 arcsec resolution replica x-ray optics for AXAF-S
Abstract
This document describes a proposal for a replica X-ray optics to be developed in Italy for NASA's X-ray spectroscopy mission AXAF-S. The program is based on state of the art technology for the production of replica X-ray optics. On the basis of the experience with the Jet-X mirror shells (to be flown on the Spectrum X-(gamma) satellite), a spatial resolution of 15 - 30 arcsec half power diameter (HPD) can be achieved for the AXAF-S optics. The characteristics of the proposed optical system are described and its performances evaluated by using the current baseline configuration for the array of X-ray calorimeters in the focal plane. The impact of the proposed replica X-ray optics is briefly outlined and a comparison with foil optics (1 - 3 arcmin HPD resolution) is carried out.
Proc. SPIE Vol. 2011, p. 149-160, Multilayer and Grazing Incidence X-Ray/EUV Optics II, Richard B. Hoover; Arthur B. Walker; Eds.Feb 1994 - Evaluation of Source Counts and Upper Limits in Crowded ROSAT PSPC Fields
Abstract
Not Available
The Soft X-ray Cosmos, Proceedings of the ROSAT Science Symposium held College Park, MD, November 8-10, 1993. AIP Conference Proceedings #313, New York: American Institute of Physics (AIP), 1994, edited by Eric M. Schlegel and Robert Petre., p.239n/a 1994 - Simulated PSPC Spectral Fits of Coronal X-ray Sources
Abstract
Not Available
The Soft X-ray Cosmos, Proceedings of the ROSAT Science Symposium held College Park, MD, November 8-10, 1993. AIP Conference Proceedings #313, New York: American Institute of Physics (AIP), 1994, edited by Eric M. Schlegel and Robert Petre., p.244n/a 1994 - ROSAT HRI Observations of T Tauri Star Pairs in Taurus-Auriga
Abstract
Not Available
The Soft X-ray Cosmos, Proceedings of the ROSAT Science Symposium held College Park, MD, November 8-10, 1993. AIP Conference Proceedings #313, New York: American Institute of Physics (AIP), 1994, edited by Eric M. Schlegel and Robert Petre., p.269n/a 1994 - X-ray Emission from the Vela SNR Shock Region: Spectral Fitting with a Non-Equilibrium Ionization Model
Abstract
Not Available
The Soft X-ray Cosmos, Proceedings of the ROSAT Science Symposium held College Park, MD, November 8-10, 1993. AIP Conference Proceedings #313, New York: American Institute of Physics (AIP), 1994, edited by Eric M. Schlegel and Robert Petre., p.309n/a 1994 - X-ray 0bservatiotus of Herbig Ae/Be stars
Abstract
Not Available
The nature and evolutionary status of Herbig Ae/Be stars. Astronomical Society of the Pacific Conference Series, Vol. 62. Proceedings of the First International Meeting held in Amsterdam, 26-29 October 1993, San Francisco: Astronomical Society of the Pacific (ASP), |c1994, edited by Pik Sin The, Mario R. Perez, and Edward P. J. Van den Heuvel, p.291n/a 1994 - ROSAT observations of the Herbig stars HD 200775, BD +61 degrees 154 and VV SER
Abstract
Not Available
The nature and evolutionary status of Herbig Ae/Be stars. Astronomical Society of the Pacific Conference Series, Vol. 62. Proceedings of the First International Meeting held in Amsterdam, 26-29 October 1993, San Francisco: Astronomical Society of the Pacific (ASP), |c1994, edited by Pik Sin The, Mario R. Perez, and Edward P. J. Van den Heuvel, p.301n/a 1994 - X-Ray and H alpha Observations of Rapidly Rotating Pre-Main-Sequence Stars
Abstract
Not Available
Cool Stars; Stellar Systems; and the Sun; Eighth Cambridge Workshop. Astronomical Society of the Pacific Conference Series, Vol. 64; Proceedings of the 8th Cambridge Workshop; held in Athens, Georgia; October 11-14; 1993; San Francisco: Astronomical Society of the Pacific (ASP); |c1994; edited by Jean-Pierre Caillault, p.71n/a 1994 - An X-Ray Study of a Coeval Star Population: A Deep ROSAT Observation of the Core of the Pleiades Cluster
Abstract
Not Available
Cool Stars; Stellar Systems; and the Sun; Eighth Cambridge Workshop. Astronomical Society of the Pacific Conference Series, Vol. 64; Proceedings of the 8th Cambridge Workshop; held in Athens, Georgia; October 11-14; 1993; San Francisco: Astronomical Society of the Pacific (ASP); |c1994; edited by Jean-Pierre Caillault, p.110n/a 1994 - A Variability Study of X-Ray Emission of Pleiades Stars
Abstract
Not Available
Cool Stars; Stellar Systems; and the Sun; Eighth Cambridge Workshop. Astronomical Society of the Pacific Conference Series, Vol. 64; Proceedings of the 8th Cambridge Workshop; held in Athens, Georgia; October 11-14; 1993; San Francisco: Astronomical Society of the Pacific (ASP); |c1994; edited by Jean-Pierre Caillault, p.140n/a 1994 - The Correlation Between Lithium and Activity in Late-Type Main Sequence Stars
Abstract
Not Available
Cool Stars; Stellar Systems; and the Sun; Eighth Cambridge Workshop. Astronomical Society of the Pacific Conference Series, Vol. 64; Proceedings of the 8th Cambridge Workshop; held in Athens, Georgia; October 11-14; 1993; San Francisco: Astronomical Society of the Pacific (ASP); |c1994; edited by Jean-Pierre Caillault, p.276n/a 1994 - The Study of the Stellar Activity-Age Correlation in Field Star Samples Through Lithium Abundance Studies
Abstract
Not Available
Frontiers of Space and Ground-Based Astronomy: The Astrophysics of the 21st century, Dordrecht, Boston: Kluwer Academic Publishers, |c1994, edited by W. Wamsteker, Malcolm. S. Longair, and Y. Kondo. Astrophysics and Space Science Library, Vol. 187, p.589n/a 1994
1993
- Stellar Coronal Emission: What We Have Learned from Pre-ROSAT Observations
Abstract
Not Available
Physics of Solar and Stellar Coronae: G. S. Vaiana Memorial Symposium, held 22-26 June, 1992, in Palermo, Italy. Edited by Jeffrey L. Linsky and Salvatore Serio. Astrophysics and Space Science Library, Vol. 183. Published by Kluwer Academoc Publishers, P. O. Box 17, 3300 AA Dordrecht, The Netherlands, 1993., p.211n/a 1993 - The G. S. Vaiana X-ray Astronomy Calibration and Testing (XACT) Facility
Abstract
Not Available
Physics of Solar and Stellar Coronae: G. S. Vaiana Memorial Symposium, held 22-26 June, 1992, in Palermo, Italy. Edited by Jeffrey L. Linsky and Salvatore Serio. Astrophysics and Space Science Library, Vol. 183. Published by Kluwer Academoc Publishers, P. O. Box 17, 3300 AA Dordrecht, The Netherlands, 1993., p.275n/a 1993 - Coronal Emission at the Low Mass End: Results from an Optical-Selected Sample of Nearby K and M Stars Surveyed with the EINSTEIN Observatory
Abstract
Not Available
Physics of Solar and Stellar Coronae: G. S. Vaiana Memorial Symposium, held 22-26 June, 1992, in Palermo, Italy. Edited by Jeffrey L. Linsky and Salvatore Serio. Astrophysics and Space Science Library, Vol. 183. Published by Kluwer Academoc Publishers, P. O. Box 17, 3300 AA Dordrecht, The Netherlands, 1993., p.279n/a 1993 - An Analysis of the Stellar Content of the EINSTEIN Extended Medium Sensitivity Survey Using Age-Homogeneous X-ray Luminosity Functions
Abstract
Not Available
Physics of Solar and Stellar Coronae: G. S. Vaiana Memorial Symposium, held 22-26 June, 1992, in Palermo, Italy. Edited by Jeffrey L. Linsky and Salvatore Serio. Astrophysics and Space Science Library, Vol. 183. Published by Kluwer Academoc Publishers, P. O. Box 17, 3300 AA Dordrecht, The Netherlands, 1993., p.287n/a 1993 - X-ray and UV Emission from Post-Main-Sequence Stars: The Connection between Surface Activity and Evolution
Abstract
Not Available
Physics of Solar and Stellar Coronae: G. S. Vaiana Memorial Symposium, held 22-26 June, 1992, in Palermo, Italy. Edited by Jeffrey L. Linsky and Salvatore Serio. Astrophysics and Space Science Library, Vol. 183. Published by Kluwer Academoc Publishers, P. O. Box 17, 3300 AA Dordrecht, The Netherlands, 1993., p.291n/a 1993 - Lithium Abundance Determination in EMSS Stellar Sources
Abstract
Not Available
Physics of Solar and Stellar Coronae: G. S. Vaiana Memorial Symposium, held 22-26 June, 1992, in Palermo, Italy. Edited by Jeffrey L. Linsky and Salvatore Serio. Astrophysics and Space Science Library, Vol. 183. Published by Kluwer Academoc Publishers, P. O. Box 17, 3300 AA Dordrecht, The Netherlands, 1993., p.303n/a 1993 - Soft X-ray Emission and Spectra of Evolved Stars from ROSAT Pointed Observations
Abstract
Not Available
Physics of Solar and Stellar Coronae: G. S. Vaiana Memorial Symposium, held 22-26 June, 1992, in Palermo, Italy. Edited by Jeffrey L. Linsky and Salvatore Serio. Astrophysics and Space Science Library, Vol. 183. Published by Kluwer Academoc Publishers, P. O. Box 17, 3300 AA Dordrecht, The Netherlands, 1993., p.365n/a 1993 - Data analysis systems and databases in astronomy: the case of the OAP-CfA Einstein stellar X-ray database
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 64, p.125n/a 1993 - A tool for X-Ray sky simulations.
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 64, p.385n/a 1993 - The G. S. Vaiana X-ray astronomy calibration and testing (XACT) facility
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 64, p.633n/a 1993 - A multi-color method for the classification of low resolution X-ray spectra
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 64, p.637n/a 1993 - High sensitivity ROSAT observations of the Pleiades
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 64, p.695n/a 1993 - Sky simulations for X-ray telescopes
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 64, p.716n/a 1993 - Parallel computations in astrophysics: using a workstation array
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 64, p.1011n/a 1993 - Evolution of stellar activity in early post-main-sequence phases
Abstract
Not Available
Inside the stars; Proceedings of the 137th IAU Colloquium, Univ. of Vienna, Austria, Apr. 13-18, 1992. Astronomical Society of the Pacific (ASP Conference Series. Vol. 40), Edited by Werner W. Weiss and Annie Baglin, 1993, p.648Jan 1993
1992
- A Comparisons Between Model Predicted and Observed Coronal Stellar Counts from the X-Ray Flux-Limited EMSS
Abstract
Not Available
Frontiers Science Series, Proceedings of the Yamada Conference XXVIII, held April 8-12, 1991, Aichi Sangyou Boueki Kaikan (Aichi Trade Center). Nagoya, Edited by Yasuo Tanaka and Katsuji Koyama. Japan, Tokyo, Japan: Universal Academy Press, 1992., p.273n/a 1992 - Coronal emission and stellar evolution
Abstract
Results of stellar X-ray astronomy based on the Einstein and Exosat observations are reviewed together with preliminary results from Rosat. Attention is given to the influence of stellar evolution on the X-ray emission from normal stars, the contribution of normal star X-ray emission to the X-ray makeup of the Galaxy, the stellar contribution to the diffuse soft X-ray background, and the origin of the Galactic Ridge. Particular consideration is given to methods for predicting the stellar coronal source number counts.
Società Astronomica Italiana, Memorie (ISSN 0037-8720), vol. 63, no. 3-4, p. 545-562.n/a 1992 - Results from an extensive Einstein Observatory survey of B-type stars
Abstract
A summary is presented of the results of an analysis of a recently compiled catalog of the 0.16-4.0 keV emission from 1545 B-type stars, taken from the Smithsonian-Astrophysical-Observatory Star Catalog (Roman and Warren, 1984), falling in the about 10-percent of the sky surveyed with the Einstein Observatory imaging proportional counter. The available information made it possible to identify a pseudovolume-limited sample containing about 54 percent of the surveyed stars and 87.5 percent of the detected ones.
Società Astronomica Italiana, Memorie (ISSN 0037-8720), vol. 63, no. 3-4, p. 739-742.n/a 1992 - X-ray emission at low-mass end of the MS – Results from an extensive Einstein Observatory survey
Abstract
The 0.16-4.0 keV X-ray emission from K and M stars of luminosity classes IV, V, and VI within 25 parsec of the sun was measured using all available Einstein Observatory IPC data and a critical compilation of catalogued optical data. Fifty four of 88 stars were detected, 70 of 138 M stars with Mv less than 13.4 (corresponding to M6) and 15 or 31 fainter M stars. The surveyed stars were grouped, on the basis of U, V, W space velocity components, into old-disk, young-disk, and halo population stars. Then, a subsample was selected which is statistically representative of the population of K and M stars in the solar neighborhood, on the basis of which unbiased Maximum Likelihood X-ray luminosity functions were constructed for K, early M, and late M stars. The investigation revealed a decrease of X-ray luminosity with increasing stellar age in the range of ages of disk population stars.
Società Astronomica Italiana, Memorie (ISSN 0037-8720), vol. 63, no. 3-4, p. 743-746.n/a 1992 - A Rosat Image of the Pleiades
Abstract
Not Available
American Astronomical Society, 179th AAS Meeting, id.94.10; Bulletin of the American Astronomical Society, Vol. 24, p.689Jan 1992
1991
- An Einstein Observatory Catalog of B-Type Stars
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 23, p.913Mar 1991
1990
- Astronomical X-ray imaging techniques.
Abstract
Contents: 1. Introduction. 2. Grazing incidence X-ray mirrors: concepts. 3. Absorption and detection of X-ray radiation: basic principles. 4. X-ray imaging detectors. 5. Data reduction and analysis. 6. Advanced data analysis technique: database approach. 7. Some scientific applications.
Seminar on Acquisition, Processing and Archiving of Astronomical Images, p. 83 - 110n/a 1990 - The Einstein Survey of O-Stars
Abstract
The authors give a brief account of some of the main results of a detailed analysis of a sample of 288 X-ray surveyed O stars.
Imaging X-Ray Astronomy. A Decade of Einstein Observatory Achievements. Editor, Martin Elvis; Publisher, Cambridge University Press, Cambridge, England, New York, 1990. LC # QB472.A1 I42 1990. ISBN # 0521381053. P.227, 1990n/a 1990 - Largescale Structures of the Soft X-Ray Background
Abstract
The Imaging Proportional Counter on board the Einstein Observatory has measured the soft (0.15 - 3.5) keV X-ray background in ≡4000 directions unevenly spaced on the sky. The authors summarize the main results of this survey.
Imaging X-Ray Astronomy. A Decade of Einstein Observatory Achievements. Editor, Martin Elvis; Publisher, Cambridge University Press, Cambridge, England, New York, 1990. LC # QB472.A1 I42 1990. ISBN # 0521381053. P.247, 1990n/a 1990 - The Einstein Observatory Stella X-Ray Database
Abstract
The authors present the motivation for and methodology followed in constructing the Einstin Observatory Stellar X-ray Database from a uniform analysis of nearly 4000 Imaging Proportional Counter fields obtained during the life of this mission. This project has been implemented using the INGRESTM database system, so that statistical analyses of the properties of detected X-ray sources are relatively easy and flexibly accomplished.
Imaging X-Ray Astronomy. A Decade of Einstein Observatory Achievements. Editor, Martin Elvis; Publisher, Cambridge University Press, Cambridge, England, New York, 1990. LC # QB472.A1 I42 1990. ISBN # 0521381053. P.313, 1990n/a 1990
1989
- Spatial and spectral features of soft diffuse X ray background seen by the Einstein observatory
Abstract
A survey of the diffuse soft X-ray background as seen directly by the Einstein Observatory Imaging Proportional Counter (IPC) is presented. A source free region of the detector 1 by 1 degree field is used. The background in the 0.16 to 3.5 keV spectral region is viewed. The data covers roughly 5 percent of the sky, with some bias in coverage towards the galactic plane. The moderate energy resolution of the IPC enables the characterization and the production of maps of the background as a function of energy within the Einstein passband. The results are compared with previous observations of the diffuse X-ray background. The implications for galactic structure and for the soft component of the extragalactic X-ray background are discussed.
In ESA, The 23rd ESLAB Symposium on Two Topics in X Ray Astronomy. Volume 2: AGN and the X Ray Background p 1005-1010 (SEE N90-25841 19-89)Nov 1989 - Nearby and bright star samples from the Einstein Observatory stellar X-ray catalog
Abstract
Results from the study of nearby and/or bright star samples are presented based on the finalized catalog of identified stellar X-ray sources observed with the Imaging Proportional Counter (IPC) onboard the Einstein Observatory. Out of a total of about 10,000 X-ray sources whose data are organized in a relational database for easily accomplishing their analysis, data of X-ray sources identified with bright or nearby optical counterparts are presented. A brief summary of the major results known to date is presented, relevant properties of the optical catalogs used for the identifications are discussed, some new spectral results are shown, and a hint is given on expectations of future X-ray observations based on present best knowledge of X-ray emission from stellar sources.
Società Astronomica Italiana, Annual Meeting, 32nd, Catania, Italy, May 4-7, 1988 Società Astronomica Italiana, Memorie (ISSN 0037-8720), vol. 60, no. 1-2, 1989, p. 125-130. Research supported by MPI, CNR, and Smithsonian Institution.n/a 1989 - An Einstein Observatory view of large-scale (equal to or greater than 1 deg) soft X-ray background structures – A status report
Abstract
The status report of an ongoing study of both spectral and intensity structures of X-ray background in the band (0.03-3.5) keV as observed by the Imaging Proportional Counter (IPC) on board the Einstein Observatory is presented. Preliminary results indicate the existence of very large scale (much larger than 1 deg) structures and suggest the presence of large-scale variation (greater than 1 deg) both in background spectrum and intensity.
Società Astronomica Italiana, Annual Meeting, 32nd, Catania, Italy, May 4-7, 1988 Società Astronomica Italiana, Memorie (ISSN 0037-8720), vol. 60, no. 1-2, 1989, p. 239-242.n/a 1989 - Independence of chromospheric active and soft X-ray flaring on the flare star EV Lacertae.
Abstract
EXOSAT observed the flare star EV Lacertae for 17 hr over 2 days in October 1985. Two flaring episodes were recorded. During a significant fraction of these observations, IUE, photometric and spectroscopic coverage was available. A 2-hour long period of X-ray flaring was observed during which there was no U-band activity and almost no chromospheric activity. On the other hand, two ΔU ≡ 1m5 optical flares produced normal chromospheric enhancements, but only a weak X-ray response. The authors suggest that these and a few other observations of stellar flares may imply the occasional existence of magnetically isolated regions in M-dwarf atmospheres.
IAU Colloquium No. 104: Solar and stellar flares. Poster papers, p. 27 - 31n/a 1989
1988
- The Einstein Observatory Stellar X-ray Database
Abstract
Not Available
From Einstein to AXAF, A Symposium to Celebrate the 10th Anniversary of the launch of HEAO-2, the Einstein Observatory, held 13-15 November, 1988 in Cambridge, MA., p.2Nov 1988 - Relationship between Optical and X-ray Properties of O-type Stars Surveyed by Einstein
Abstract
Not Available
From Einstein to AXAF, A Symposium to Celebrate the 10th Anniversary of the launch of HEAO-2, the Einstein Observatory, held 13-15 November, 1988 in Cambridge, MA., p.13Nov 1988 - Coronal Temperatures of late-type stars
Abstract
Not Available
From Einstein to AXAF, A Symposium to Celebrate the 10th Anniversary of the launch of HEAO-2, the Einstein Observatory, held 13-15 November, 1988 in Cambridge, MA., p.14Nov 1988 - An Einstein Observatory View of Large-Scale Soft X-ray Background Structures: A Status Report
Abstract
Not Available
From Einstein to AXAF, A Symposium to Celebrate the 10th Anniversary of the launch of HEAO-2, the Einstein Observatory, held 13-15 November, 1988 in Cambridge, MA., p.28Nov 1988 - The Einstein Observatory Stellar X-ray Database: an overview.
Abstract
The authors present the motivations for and the methodology followed in building the "Einstein Observatory Stellar X-ray Database" based on the uniform analysis of all Einstein Observatory Imaging Proportional Counter fields obtained during the life of the HEAO-2 mission. The database has been implemented using the INGRESTM database system, so that statistical analyses of the properties of the full detection catalog are relatively easily and flexibly accomplished. Some illustrative examples will furnish a general view both of the kind and the amount of the archived information, and of the statistical approach used in analyzing the global properties of the data.
ESO Conference Workshop Proceedings, No. 28, p. 483 - 487n/a 1988 - UNIX (TM) Experience at Osservatorio Astronomico di Paermo
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 59, p.401n/a 1988 - The Stellar X-Ray Data Base of Einstein Image Observations
Abstract
Not Available
Memorie della Società Astronomia Italiana, Vol. 59, p.465n/a 1988
1987
- An Einstein Observatory Stellar X-ray Catalog
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 19, p.1040Sep 1987 - Observations of stellar coronae
Abstract
The authors present an overview of recent stellar X-ray observations, with some discussion of the requirements for future observations. They argue that solar observations indicate that coronal X-ray emission is strongly related to surface magnetic field activity; they show that the interpretation of X-ray stellar coronal emission from late-type stars within the framework of models analogous to those developed for the solar corona is viable, and it is supported by many experimental results. The extension of this solar analogy to the early-type stars is quite questionable and remains an unsolved problem, while the working hypothesis of an X-ray phase, related to phenomena of magnetic field-related activity, as contrasted to a wind phase during the PMS evolutionary stage is suggested by the present status of observations.
IN: Circumstellar matter; Proceedings of the IAU Symposium, Heidelberg, Federal Republic of Germany, June 23-27, 1986 (A88-28451 10-90). Dordrecht, D. Reidel Publishing Co., 1987, p. 333-345. Research supported by the Ministero della Pubblica Istruzione and CNR.n/a 1987
1986
- Rapid X-ray Variability of Quiescent Late-Type Dwarfs: Implications for Coronal Heating
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 18, p.985Sep 1986
1985
- X-Ray Variability in K and M Dwarfs Observed by Einstein
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 17, p.598Mar 1985 - The relationship between X-ray luminosity and Rossby number for a sample of late-type stars.
Abstract
The authors investigate the relation between the X-ray luminosity and the ratio of rotational period to convective turn-over time (i.e. Rossby number) for a sample of stars later than F7 belonging to three groups with different ages: nearby stars, Hyades and Pleiades.
X-ray astronomyn/a 1985 - Optimized variability analysis in non periodic sources: application to X-ray emission from normal stars.
Abstract
The authors discuss the application of a χ2 method for variability analysis of aperiodic sources. By binning photon arrival times, they consider the deviation of the values of χ2 from those expected under the hypothesis of a constant source with Poisson noise, and its dependence on the binning rate and phase. They determine from this dependence an "effective" fractional variability and the dominant time scale of the variable component (if any). They use Monte Carlo simulations to check the validity of the method employed. They show applications of this method to X-ray emission from normal stars.
X-ray astronomyn/a 1985
1984
- Far-Infrared Spectrum of IRC+10216
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 16, p.484Mar 1984 - Far-Infrared and CO Observations of the W33 Complex
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 16, p.522Mar 1984 - X-Ray Survey of the Pleiades – Dependence of X-Ray Luminosity on Stellar Age
Abstract
The authors report preliminary results from an Einstein X-ray survey of the Pleiades. They have analysed a 1°×1° exposure centered over one of the more luminous stars of the cluster (20 Tau, [B7 III]). This field contains ≡62 cluster members out of a total of ≡270 stars with magnitude lower than 14m. (Hertzsprung, 1947). The authors have detected 17 distinct X-ray sources; 16 sources are identified with cluster stars within a distance less than 1arcmin.
Observational Tests of the Stellar Evolution Theory. International Astronomical Union Symposium No. 105, held in Geneva, Switzerland, September 12-16, 1983. Editors, Andre Maeder, Alvio Renzini; Publisher, D. Reidel Publishing Company, Dordrecht, The Netherlands, Boston, MA, Hingham, MA. Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, 1984. LC # QB806 .O27 1984. ISBN # 90-277-1774-5. P.101, 1984n/a 1984
1983
- X-Ray Variable Stars in the Pleiades
Abstract
Not Available
Information Bulletin on Variable Stars, No. 2449, #1.Dec 1983
1982
- The Temperature-Luminosity Dependence of Stellar X-ray Sources
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 14, p.945Sep 1982 - Variable X-ray Emission from Early-type Stars
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 14, p.945Sep 1982 - Multicolor Far-Infrared Observations of NGC 6334
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 14, p.931Sep 1982 - Il sistema di trattamento di immagini presso l
Abstract
Not Available
Memorie della Societa Astronomica Italiana, Vol. 53, No. 1, p. 115 - 129n/a 1982
1979
- Far-Infrared Observations of Regions Near W42
Abstract
Not Available
Bulletin of the American Astronomical Society, Vol. 11, p.470Mar 1979 - Transienti nella corona solare: determinazione dei parametri fisici del plasma e leggi di scala.
Abstract
Not Available
XXII Riunione della Società Astronomica Italiana, p. 165 - 166n/a 1979 - Sistema analisi dati per fotometria nel lontano infrarosso della collaborazione Palermo-Harvard. Risultati preliminari per la regione intorno a W42.
Abstract
Not Available
XXII Riunione della Società Astronomica Italiana, p. 190 - 191n/a 1979 - Rivelazione quantica di immagini ad alta risoluzione spazio-temporale: il progetto Photicon.
Abstract
Not Available
XXII Riunione della Società Astronomica Italiana, p. 237 - 238n/a 1979