Beate Stelzer
Bibliografia
Refereed
2016
- A path towards understanding the rotation-activity relation of M dwarfs with K2 mission, X-ray and UV data
Abstract
We study the relation between stellar rotation and magnetic activity for a sample of 134 bright, nearby M dwarfs observed in the Kepler Two-Wheel (K2) mission during campaigns C0 to C4. The K2 lightcurves yield photometrically derived rotation periods for 97 stars (79 of which without previous period measurement), as well as various measures for activity related to cool spots and flares. We find a clear difference between fast and slow rotators with a dividing line at a period of ~10d at which the activity level changes abruptly. All photometric diagnostics of activity (spot cycle amplitude, flare peak amplitude and residual variability after subtraction of spot and flare variations) display the same dichotomy, pointing to a quick transition between a high-activity mode for fast rotators and a low-activity mode for slow rotators. This unexplained behavior is reminiscent of a dynamo mode-change seen in numerical simulations that separates a dipolar from a multipolar regime. A substantial number of the fast rotators are visual binaries. A tentative explanation is accelerated disk evolution in binaries leading to higher initial rotation rates on the main-sequence and associated longer spin-down and activity lifetimes. We combine the K2 rotation periods with archival X-ray and UV data. X-ray, FUV and NUV detections are found for 26, 41, and 11 stars from our sample, respectively. Separating the fast from the slow rotators, we determine for the first time the X-ray saturation level separately for early- and for mid-M stars.
Monthly Notices of the Royal Astronomical Society, vol. 463, issue 2, pp. 1844-1864Dec 2016 - The Multiple Young Stellar Objects of HBC 515: An X-ray and Millimeter-wave Imaging Study in (Pre-main Sequence) Diversity
Abstract
We present Chandra X-ray Observatory and Submillimeter Array (SMA) imaging of HBC 515, a system consisting of multiple young stellar objects (YSOs). The five members of HBC 515 represent a remarkably diverse array of YSOs, ranging from the low-mass Class I/II protostar HBC 515B, through Class II and transition disk objects (HBC 515D and C, respectively), to the "diskless", intermediate- mass, pre-main sequence binary HBC 515A. Our Chandra/ACIS imaging establishes that all five components are X-ray sources, with HBC 515A - a subarcsecond-separation binary that is partially resolved by Chandra - being the dominant X-ray source. We detect an X-ray flare associated with HBC 515B. In the SMA imaging, HBC 515B is detected as a strong 1.3 mm continuum emission source; a second, weaker mm continuum source is coincident with the position of the transition disk object HBC 515C. These results strongly support the protostellar nature of HBC 515B, and firmly establish HBC 515A as a member of the rare class of relatively massive, X-ray luminous "weak-lined T Tauri stars" that are binaries and have shed their disks at very early stages of pre-MS evolution. The coexistence of two such disparate objects within a single, presumably coeval multiple YSO system highlights the influence of pre- MS star mass, binarity, and X-ray luminosity in regulating the lifetimes of circumstellar, planet-forming disks and the timescales of star-disk interactions.
Astronomy & Astrophysics, Volume 598, id.A8, 9 pp.Oct 2016 - The HADES RV Programme with HARPS-N@TNG. III. Flux-flux and activity-rotation relationships of early-M dwarfs
Abstract
(Abridged) Understanding stellar activity in M dwarfs is crucial for the physics of stellar atmospheres as well as for ongoing radial velocity exoplanet programmes. Despite the increasing interest in M dwarfs, our knowledge of the chromospheres of these stars is far from being complete. We aim to test whether the relations between activity, rotation, and stellar parameters and flux-flux relationships also hold for early-M dwarfs on the main-sequence. We analyse in an homogeneous and coherent way a well defined sample of 71 late-K/early-M dwarfs that are currently being observed in the framework of the HArps-n red Dwarf Exoplanet Survey (HADES). Rotational velocities are derived using the cross-correlation technique while emission flux excesses in the Ca II H & K and Balmer lines from Halpha up to Hepsilon are obtained by using the spectral subtraction technique. The relationships between the emission excesses and the stellar parameters are studied. Relations between pairs of fluxes of different chromospheric lines are also studied. We find that the strength of the chromospheric emission in the Ca II H & K and Balmer lines is roughly constant for stars in the M0-M3 spectral range. Our data suggest that a moderate but significant correlation between activity and rotation might be present as well as a hint of kinematically selected young stars showing higher levels of emission. We find our sample of M dwarfs to be complementary in terms of chromospheric and X-ray fluxes with those of the literature, extending the analysis of the flux-flux relationships to the very low flux domain. Our results agree with previous works suggesting that the activity-rotation-age relationship known to hold for solar-type stars also applies to early-M dwarfs. We also confirm previous findings that the field stars which deviate from the bulk of the empirical flux-flux relationships show evidence of youth.
Astronomy & Astrophysics, Volume 598, id.A27, 19 pp.Oct 2016 - The HADES RV Programme with HARPS-N@TNG IV. Time resolved analysis of the Ca ii H&K and H{\alpha} chromospheric emission of low-activity early-type M dwarfs
Abstract
M dwarfs are prime targets for planet search programs, particularly of those focused on the detection and characterization of rocky planets in the habitable zone. Understanding their magnetic activity is important because it affects our ability to detect small planets, and it plays a key role in the characterization of the stellar environment. We analyze observations of the Ca II H&K and H{\alpha} lines as diagnostics of chromospheric activity for low-activity early-type M dwarfs. We analyze the time series of spectra of 71 early-type M dwarfs collected for the HADES project for planet search purposes. The HARPS-N spectra provide simultaneously the H&K doublet and the H{\alpha} line. We develop a reduction scheme able to correct the HARPS-N spectra for instrumental and atmospheric effects, and to provide flux-calibrated spectra in units of flux at the stellar surface. The H&K and H{\alpha} fluxes are compared with each other, and their variability is analyzed. We find that the H and K flux excesses are strongly correlated with each other, while the H{\alpha} flux excess is generally less correlated with the H&K doublet. We also find that H{\alpha} emission does not increase monotonically with the H&K line flux, showing some absorption before being filled in by chromospheric emission when H&K activity increases. Analyzing the time variability of the emission fluxes, we derive a tentative estimate of the rotation period (of the order of a few tens of days) for some of the program stars, and the typical lifetime of chromospheric active regions (a few stellar rotations). Our results are in good agreement with previous studies. In particular, we find evidence that the chromospheres of early-type M dwarfs could be characterized by different filaments coverage, affecting the formation mechanism of the H{\alpha} line. We also show that chromospheric structure is likely related to spectral type.
Astronomy & Astrophysics, Volume 598, id.A28, 14 pp.Oct 2016 - Spectroscopic signatures of magnetospheric accretion in Herbig Ae/Be stars. I. The case of HD 101412
Abstract
Context. Models of magnetically-driven accretion and outflows reproduce many observational properties of T Tauri stars. This concept is not well established for the more massive Herbig Ae/Be stars. Aims: We intend to examine the magnetospheric accretion in Herbig Ae/Be stars and search for rotational modulation using spectroscopic signatures, in this first paper concentrating on the well-studied Herbig Ae star HD 101412. Methods: We used near-infrared spectroscopic observations of the magnetic Herbig Ae star HD 101412 to test the magnetospheric character of its accretion disk/star interaction. We reduced and analyzed 30 spectra of HD 101412, acquired with the CRIRES and X-shooter spectrographs installed at the VLT (ESO, Chile). The spectroscopic analysis was based on the He iλ10 830 and Paγ lines, formed in the accretion region. Results: We found that the temporal behavior of these diagnostic lines in the near-infrared spectra of HD 101412 can be explained by rotational modulation of line profiles generated by accreting gas with a period P = 20.53d±1.68d. The discovery of this period, about half of the magnetic rotation period Pm = 42.076d previously determined from measurements of the mean longitudinal magnetic field, indicates that the accreted matter falls onto the star in regions close to the magnetic poles intersecting the line-of-sight two times during the rotation cycle. We intend to apply this method to a larger sample of Herbig Ae/Be stars. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 087.C-0124(A), 088.C-0218(A,B,C,E), 090.C-0331(A), and 092.C-0126(A).
Astronomy & Astrophysics, Volume 592, id.A50, 8 pp.Jul 2016 - M Stars in the TW Hya Association: Stellar X-Rays and Disk Dissipation
Abstract
To investigate the potential connection between the intense X-ray emission from young low-mass stars and the lifetimes of their circumstellar planet-forming disks, we have compiled the X-ray luminosities (L X ) of M stars in the ˜8 Myr old TW Hya Association (TWA) for which X-ray data are presently available. Our investigation includes analysis of archival Chandra data for the TWA binary systems TWA 8, 9, and 13. Although our study suffers from poor statistics for stars later than M3, we find a trend of decreasing {L}X/{L}{bol} with decreasing T eff for TWA M stars, wherein the earliest-type (M0-M2) stars cluster near {log}({L}X/{L}{bol})≈ -3.0 and then {log}({L}X/{L}{bol}) decreases, and its distribution broadens, for types M4 and later. The fraction of TWA stars that display evidence for residual primordial disk material also sharply increases in this same (mid-M) spectral type regime. This apparent anticorrelation between the relative X-ray luminosities of low-mass TWA stars and the longevities of their circumstellar disks suggests that primordial disks orbiting early-type M stars in the TWA have dispersed rapidly as a consequence of their persistent large X-ray fluxes. Conversely, the disks orbiting the very lowest-mass pre-MS stars and pre-MS brown dwarfs in the Association may have survived because their X-ray luminosities and, hence, disk photoevaporation rates are very low to begin with, and then further decline relatively early in their pre-MS evolution.
The Astronomical Journal, Volume 152, Issue 1, article id. 3, 11 pp. (2016).Jul 2016 - Evidence for variable, correlated X-ray and optical/IR extinction towards the nearby, pre-main-sequence binary TWA 30
Abstract
We present contemporaneous XMM-Newton X-ray and ground-based optical/near-IR spectroscopic observations of the nearby (D ≈ 42 pc), low-mass (mid-M) binary system TWA 30A and 30B. The components of this wide (separation ˜3400 au) binary are notable for their nearly edge-on disc viewing geometries, high levels of variability, and evidence for collimated stellar outflows. We obtained XMM-Newton X-ray observations of TWA 30A and 30B in 2011 June and July, accompanied (respectively) by Infrared Telescope Facility SpeX (near-IR) and VLT XSHOOTER (visible/near-IR) spectroscopy obtained within ˜20 h of the X-ray observations. TWA 30A was detected in both XMM-Newton observations at relatively faint intrinsic X-ray luminosities (LX ˜ 8 × 1027 erg s-1) compared to stars of similar mass and age. The intrinsic (0.15-2.0 keV) X-ray luminosities measured in 2011 had decreased by a factor 20-100 relative to a 1990 (ROSAT) X-ray detection. TWA 30B was not detected, and we infer an upper limit on its X-ray Luminosity of LX ≲ 3.0 × 1027 erg s-1. We measured a decrease in visual extinction towards TWA 30A (from AV ≈ 14.9 to AV ≈ 4.7) between the two 2011 observing epochs, and we find evidence for a corresponding significant decrease in X-ray absorbing column (NH). The apparent correlated change in AV and NH is suggestive of variable obscuration of the stellar photosphere by disc material composed of both gas and dust. However, in both observations, the inferred NH to AV ratio is lower than that typical of the interstellar medium, suggesting that the disc is either depleted of gas or is deficient in metals in the gas phase.
Monthly Notices of the Royal Astronomical Society, Volume 459, Issue 2, p.2097-2105Jun 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
- First X-ray detection of the young variable V1180 Cassiopeiae
Abstract
Context. V1180 Cas is a young variable that has shown strong photometric fluctuations (ΔI ~ 6 mag) in the recent past, which have been attributed to events of enhanced accretion. The source has entered a new high-brightness state in September 2013, which we have previously analysed through optical and near-infrared spectroscopy. Aims: To investigate the current active phase of V1180 Cas, we performed observations with the Chandra satellite aimed at studying the X-ray emission from the object and its connection to accretion episodes. Methods: Chandra observations were performed in early August 2014. Complementary JHK photometry and J-band spectroscopy were taken at our Campo Imperatore facility to relate the X-ray and near-infrared emission from the target. Results: We observe a peak of X-ray emission at the nominal position of V1180 Cas and estimate that the confidence level of the detection is about 3σ. The observed signal corresponds to an X-ray luminosity LX(0.5-7 keV) in the range 0.8 ÷ 2.2 × 1030 erg s-1. Based on the relatively short duration of the dim states in the light curve and on stellar luminosity considerations, we explored the possibility that the brightness minima of V1180 Cas are driven by extinction variations. From the analysis of the spectral energy distribution of the high state we infer a stellar luminosity of 0.8-0.9 L⊙ and find that the derived LX is comparable to the average X-ray luminosity values observed in T Tauri objects. Moreover, the X-ray luminosity appears to be lower than the X-ray emission levels around 5 × 1030 ÷ 1 × 1031 erg s-1 detected at outbursts in similar low-mass objects. Conclusions: Our analysis suggests that at least part of the photometric fluctuations of V1180 Cas might be extinction effects rather than the result of accretion excess emission. However, because the source displays spectral features indicative of active accretion, we speculate that its photometric variations might be the result of a combination of accretion-induced and extinction-driven effects, as suggested for other young variables, such as V1184 Tau and V2492 Cyg.
Astronomy & Astrophysics, Volume 584, id.A21, 6 pp.Dec 2015 - Variability of young stellar objects: Accretion, disks, outflows, and magnetic activity
Abstract
Variability is a key characteristic of young stars. Two major origins may be distinguished: a scaled-up version of the magnetic activity seen on main-sequence stars and various processes related to circumstellar disks, accretion, and outflows.
Astronomische Nachrichten, Vol.336, Issue 5, p.493Jun 2015 - The Deepest Constraints on Radio and X-Ray Magnetic Activity in Ultracool Dwarfs from WISE J104915.57-531906.1
Abstract
We report upper limits to the radio and X-ray emission from the newly discovered ultracool dwarf binary WISE J104915.57-531906.1 (Luhman 16AB). As the nearest ultracool dwarf binary (2 pc), its proximity offers a hefty advantage to studying plasma processes in ultracool dwarfs which are more similar in gross properties (radius, mass, temperature) to the solar system giant planets than stars. The radio and X-ray emission upper limits from the Australia Telescope Compact Array and Chandra observations, each spanning multiple rotation periods, provide the deepest fractional radio and X-ray luminosities to date on an ultracool dwarf, with log ({{L}r,ν }/{{L}bol})[H{{z}-1}]\lt -18.1 (5.5 GHz), log ({{L}r,ν }/{{L}bol})[H{{z}-1}]\lt -17.9 (9 GHz), and log ({{L}x}/{{L}bol})\lt -5.7. While the radio upper limits alone do not allow for a constraint on the magnetic field strength, we limit the size of any coherently emitting region in our line of sight to less than 0.2% of the radius of one of the brown dwarfs. Any source of incoherent emission must span less than about 20% of the brown dwarf radius, assuming magnetic field strengths of a few tens to a few hundred Gauss. The fast rotation and large amplitude photometric variability exhibited by the T dwarf in the Luhman 16AB system are not accompanied by enhanced nonthermal radio emission or enhanced heating to coronal temperatures, as observed on some higher mass ultracool dwarfs, confirming the expected decoupling of matter and magnetic field in cool neutral atmospheres.
The Astrophysical Journal Letters, Volume 805, Issue 1, article id. L3, 5 pp. (2015).May 2015 - Empirical Determination of Einstein A-coefficient Ratios of Bright [Fe II] Lines
Abstract
The Einstein spontaneous rates (A-coefficients) of Fe+ lines have been computed by several authors with results that differ from each other by up to 40%. Consequently, models for line emissivities suffer from uncertainties that in turn affect the determination of the physical conditions at the base of line excitation. We provide an empirical determination of the A-coefficient ratios of bright [Fe II] lines that would represent both a valid benchmark for theoretical computations and a reference for the physical interpretation of the observed lines. With the ESO-Very Large Telescope X-shooter instrument between 3000 Å and 24700 Å, we obtained a spectrum of the bright Herbig-Haro object HH 1. We detect around 100 [Fe II] lines, some of which with a signal-to-noise ratios >=100. Among these latter lines, we selected those emitted by the same level, whose dereddened intensity ratios are direct functions of the Einstein A-coefficient ratios. From the same X-shooter spectrum, we got an accurate estimate of the extinction toward HH 1 through intensity ratios of atomic species, H I recombination lines and H2 ro-vibrational transitions. We provide seven reliable A-coefficient ratios between bright [Fe II] lines, which are compared with the literature determinations. In particular, the A-coefficient ratios involving the brightest near-infrared lines (λ12570/λ16440 and λ13209/λ16440) are in better agreement with the predictions by the Quinet et al. relativistic Hartree-Fock model. However, none of the theoretical models predict A-coefficient ratios in agreement with all of our determinations. We also show that literature data of near-infrared intensity ratios better agree with our determinations than with theoretical expectations. Based on observations collected with X-shooter at the Very Large Telescope on Cerro Paranal (Chile), operated by the European Southern Observatory (ESO). Program ID: 092.C-0058(A).
The Astrophysical Journal, Volume 798, Issue 1, article id. 33, 5 pp. (2015).Jan 2015
2014
- Accretion-ejection connection in the young brown dwarf candidate ISO-ChaI 217
Abstract
As the number of observed brown dwarf outflows is growing it is important to investigate how these outflows compare to the well-studied jets from young stellar objects. A key point of comparison is the relationship between outflow and accretion activity and in particular the ratio between the mass outflow and accretion rates (M⊙out/M⊙acc). The brown dwarf candidate ISO-ChaI 217 was discovered by our group, as part of a spectro-astrometric study of brown dwarfs, to be driving an asymmetric outflow with the blue-shifted lobe having a position angle of ~20°. The aim here is to further investigate the properties of ISO-ChaI 217, the morphology and kinematics of its outflow, and to better constrain M⊙out/M⊙acc. The outflow is spatially resolved in the [S ii] λλ6716,6731 lines and is detected out to ~1.̋6 in the blue-shifted lobe and 1
Astronomy & Astrophysics, Volume 570, id.A59, 9 pp.Oct 2014 - X-shooter spectroscopy of young stellar objects. V. Slow winds in T Tauri stars
Abstract
Disks around T Tauri stars are known to lose mass, as best shown by the profiles of the forbidden emission lines of low-ionization species. At least two separate kinematic components have been identified, one characterized by velocity shifts of tens to hundreds of km s-1 (HVC) and one with a much lower velocity of a few km s-1 (LVC). The HVC are convincingly associated to the emission of jets, but the origin of the LVC is still unknown. In this paper we analyze the forbidden line spectrum of a sample of 44 mostly low-mass young stars in Lupus and σ Ori observed with the X-shooter ESO spectrometer. We detect forbidden line emission of O i, O ii, S ii, N i, and N ii, and characterize the line profiles as LVC, blueshifted HVC, and redshifted HVC. We focus our study on the LVC. We show that there is a good correlation between line luminosity and both Lstar and the accretion luminosity (or the mass accretion rate) over a large interval of values (Lstar~ 10-2-1 L⊙; Lacc~ 10-5-10-1 L⊙; Ṁacc~ 10-11 - 10-7 M⊙/yr). The lines show the presence of a slow wind (Vpeak 108 cm-3), warm (T ~ 5000-10 000 K), mostly neutral. We estimate the mass of the emitting gas and provide a value for the maximum volume it occupies. Both quantities increase steeply with the stellar mass, from ~ 10-12 M⊙ and ~0.01 AU3 for Mstar~ 0.1 M⊙, to ~ 3 × 10-10 M⊙ and ~1 AU3 for Mstar~ 1 M⊙, respectively. These results provide quite stringent constraints to wind models in low-mass young stars, that need to be explored further. Based on observations collected at the European Souther Observatory at Paranal, under programs 084.C-0269(A), 085.C-0238(A), 086.C-0173(A), 087.C-0244(A) and 089.C-0143(A).Appendices are available in electronic form at http://www.aanda.org
Astronomy & Astrophysics, Volume 569, id.A5, 22 pp.Sep 2014 - Erratum: The UV and X-ray activity of the M dwarfs within 10 pc of the Sun
Abstract
Not Available
Monthly Notices of the Royal Astronomical Society, Volume 442, Issue 1, p.343-346Jul 2014 - ESO-Hα 574 and Par-Lup 3-4 jets: Exploring the spectral, kinematical, and physical properties
Abstract
In this paper a comprehensive analysis of VLT/X-Shooter observations of two jet systems, namely ESO-Hα 574 a K8 classical T Tauri star and Par-Lup 3-4 a very low mass (0.13 M⊙) M5 star, is presented. Both stars are known to have near-edge on accretion disks. A summary of these first X-shooter observations of jets was given in a 2011 letter. The new results outlined here include flux tables of identified emission lines, information on the morphology, kinematics and physical conditions of both jets and, updated estimates of Ṁout/Ṁacc. Asymmetries in the ESO-Hα 574 flow are investigated while the Par-Lup 3-4 jet is much more symmetric. The density, temperature, and therefore origin of the gas traced by the Balmer lines are investigated from the Balmer decrements and results suggest an origin in a jet for ESO-Hα 574 while for Par-Lup 3-4 the temperature and density are consistent with an accretion flow. Ṁacc is estimated from the luminosity of various accretion tracers. For both targets, new luminosity relationships and a re-evaluation of the effect of reddening and grey extinction (due to the edge-on disks) allows for substantial improvements on previous estimates of Ṁacc. It is found that log(Ṁacc) = -9.15 ± 0.45M⊙ yr-1 and -9.30 ± 0.27M⊙ yr-1 for ESO-Hα 574 and Par-Lup 3-4 respectively. Additionally, the physical conditions in the jets (electron density, electron temperature, and ionisation) are probed using various line ratios and compared with previous determinations from iron lines. The results are combined with the luminosity of the [SII]λ6731 line to derive Ṁout through a calculation of the gas emissivity based on a 5-level atom model. As this method for deriving Ṁout comes from an exact calculation based on the jet parameters (measured directly from the spectra) rather than as was done previously from an approximate formula based on the value of the critical density at an assumed unknown temperature, values of Ṁout are far more accurate. Overall the accuracy of earlier measurements of Ṁout/Ṁacc is refined and Ṁout/Ṁacc = 0.5 (+1.0)(- 0.2) and 0.3 (+0.6)(- 0.1) for the ESO-Hα 574 red and blue jets, respectively, and 0.05 (+0.10)(- 0.02) for both the Par-Lup 3-4 red and blue jets. While the value for the total (two-sided) Ṁout/Ṁacc in ESO-Hα 574 lies outside the range predicted by magneto-centrifugal jet launching models, the errors are large and the effects of veiling and scattering on extinction measurements, and therefore the estimate of Ṁacc, should also be considered. ESO-Hα 574 is an excellent case study for understanding the impact of an edge-on accretion disk on the observed stellar emission. The improvements in the derivation of Ṁout/Ṁacc means that this ratio for Par-Lup 3-4 now lies within the range predicted by leading models, as compared to earlier measurements for very low mass stars. Par-Lup 3-4 is one of a small number of brown dwarfs and very low mass stars which launch jets. Therefore, this result is important in the context of understanding how Ṁout/Ṁacc and, thus, jet launching mechanisms for the lowest mass jet drivingsources, compare to the case of the well-studied low mass stars. Based on Observations collected with X-Shooter and UVES at the Very Large Telescope on Cerro Paranal (Chile), operated by the European Southern Observatory (ESO). Program ID
Astronomy & Astrophysics, Volume 565, id.A80, 22 pp.May 2014 - X-shooter spectroscopy of young stellar objects. IV. Accretion in low-mass stars and substellar objects in Lupus
Abstract
We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from ~0.03 to ~1.2 M⊙, but mostly with 0.1 M⊙<M⋆< 0.5 M⊙. Our aim is twofold: firstly, to analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (Lacc), and in turn the accretion rate (Ṁacc), was derived by modelling the excess emission from the UV to the near-infrared as the continuum emission of a slab of hydrogen. We computed the flux and luminosity (Lline) of many emission lines of H , He , and Ca ii, observed simultaneously in the range from ~330 nm to 2500 nm. The luminosity of all the lines is well correlated with Lacc. We provide empirical relationships between Lacc and the luminosity of 39 emission lines, which have a lower dispersion than relationships previously reported in the literature. Our measurements extend the Paβ and Brγ relationships to Lacc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies of measuring Lacc and Ṁacc yield significantly different results: Hα line profile modelling may underestimate Ṁacc by 0.6 to 0.8 dex with respect to Ṁacc derived from continuum-excess measures. These differences may explain the probably spurious bi-modal relationships between Ṁacc and other YSOs properties reported in the literature. We derived Ṁacc in the range 2 × 10-12-4 × 10-8 M⊙ yr-1 and conclude that Ṁacc ∝ M⋆1.8(±0.2), with a dispersion lower by a factor of about 2 than in previous studies. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude in Ṁacc, confirming previous suggestions that the geometry of the accretion flow controls the rate at which the disc material accretes onto the central star. Based on observations collected at the European Souther Observatory at Paranal, under programs 084.C-0269(A), 085.C-0238(A), 086.C-0173(A), 087.C-0244(A) and 089.C-0143(A).Tables 1-4 and Appendices are available in electronic form at http://www.aanda.org
Astronomy & Astrophysics, Volume 561, id.A2, 43 pp.Jan 2014
2013
- The Diagnostic Potential of Fe Lines Applied to Protostellar Jets
Abstract
We investigate the diagnostic capabilities of iron lines for tracing the physical conditions of shock-excited gas in jets driven by pre-main sequence stars. We have analyzed the 3000-25000 Å, X-shooter spectra of two jets driven by the pre-main sequence stars ESO-Hα 574 and Par-Lup 3-4. Both spectra are very rich in [Fe II] lines over the whole spectral range; in addition, lines from [Fe III] are detected in the ESO-Hα 574 spectrum. Non-local thermal equilibrium codes solving the equations of the statistical equilibrium along with codes for the ionization equilibrium are used to derive the gas excitation conditions of electron temperature and density and fractional ionization. An estimate of the iron gas-phase abundance is provided by comparing the iron lines emissivity with that of neutral oxygen at 6300 Å. The [Fe II] line analysis indicates that the jet driven by ESO-Hα 574 is, on average, colder (T e ~ 9000 K), less dense (n e ~ 2 × 104 cm-3), and more ionized (x e ~ 0.7) than the Par-Lup 3-4 jet (T e ~ 13,000 K, n e ~ 6 × 104 cm-3, x e < 0.4), even if the existence of a higher density component (n e ~ 2 × 105 cm-3) is probed by the [Fe III] and [Fe II] ultra-violet lines. The physical conditions derived from the iron lines are compared with shock models suggesting that the shock at work in ESO-Hα 574 is faster and likely more energetic than the Par-Lup 3-4 shock. This latter feature is confirmed by the high percentage of gas-phase iron measured in ESO-Hα 574 (50%-60% of its solar abundance in comparison with less than 30% in Par-Lup 3-4), which testifies that the ESO-Hα 574 shock is powerful enough to partially destroy the dust present inside the jet. This work demonstrates that a multiline Fe analysis can be effectively used to probe the excitation and ionization conditions of the gas in a jet without any assumption on ionic abundances. The main limitation on the diagnostics resides in the large uncertainties of the atomic data, which, however, can be overcome through a statistical approach involving many lines. Based on observations collected with X-shooter at the Very Large Telescope on Cerro Paranal (Chile), operated by the European Southern Observatory (ESO). Program ID: 085.C-0238(A).
The Astrophysical Journal, Volume 778, Issue 1, article id. 71, 13 pp. (2013).Nov 2013 - X-shooter spectroscopy of young stellar objects. III. Photospheric and chromospheric properties of Class III objects
Abstract
Context. Traditionally, the chromospheres of late-type stars are studied through their strongest emission lines, Hα and Ca ii HK emission. Our knowledge on the whole emission line spectrum is more elusive as a result of the limited spectral range and sensitivity of most available spectrographs. Aims: We intend to reduce this gap with a comprehensive spectroscopic study of the chromospheric emission line spectrum of a sample of non-accreting pre-main sequence stars (Class III sources). Methods: We analyzed X-shooter/VLT spectra of 24 Class III sources from three nearby star-forming regions (σ Orionis, Lupus III, and TW Hya). We determined the effective temperature, surface gravity, rotational velocity, and radial velocity by comparing the observed spectra with synthetic BT-Settl model spectra. We investigated in detail the emission lines emerging from the stellar chromospheres and combined these data with archival X-ray data to allow for a comparison between chromospheric and coronal emissions. Results: For some objects in the sample the atmospheric and kinematic parameters are presented here for the first time. The effective temperatures are consistent with those derived for the same stars from an empirical calibration with spectral types. Small differences in the surface gravity found between the stars can be attributed to differences in the average age of the three star-forming regions. The strength of lithium absorption and radial velocities confirm the young age of all but one object in the sample (Sz 94). Both X-ray and Hα luminosity as measured in terms of the bolometric luminosity are independent of the effective temperature for early-M stars but decline toward the end of the spectral M sequence. For the saturated early-M stars the average emission level is almost one dex higher for X-rays than for Hα: log (Lx/Lbol) = -2.85 ± 0.36 vs. log (LHα/Lbol) = -3.72 ± 0.21. When all chromospheric emission lines (including the Balmer series up to H11, Ca ii HK, the Ca ii infrared triplet, and several He i lines) are summed up the coronal flux still dominates that of the chromosphere, typically by a factor 2-5. Flux-flux relations between activity diagnostics that probe different atmospheric layers (from the lower chromosphere to the corona) separate our sample of active pre-main sequence stars from the bulk of field M dwarfs studied in the literature. Flux ratios between individual optical emission lines show a smooth dependence on the effective temperature. The Balmer decrements can roughly be reproduced by an NLTE radiative transfer model devised for another young star of similar age. Future, more complete chromospheric model grids can be tested against this data set. Based on observations collected at the Very Large Telescope of the European Southern Observatory under programs 084.C-0269, 085.C-0238, 086.C-0173, 087.C-0244, and 089.C-0143.Tables 2-4 are available in electronic form at http://www.aanda.org
Astronomy & Astrophysics, Volume 558, id.A141, 16 pp.Oct 2013 - ιHorologi, the first coronal activity cycle in a young solar-like star
Abstract
Context. The shortest chromospheric (Ca ii H&K) activity cycle (1.6 yr) has been recently discovered in the young (~600 Myr) solar-like star ι Hor. Coronal X-ray activity cycles have only been discovered in a few stars other than the Sun, all of them with an older age and a lower activity level than ι Hor. Aims: We intended to find the X-ray coronal counterpart of the chromospheric cycle for ι Hor. This represents the first X-ray cycle observed in an active star, as well as the paradigm of the first coronal cycles in the life of a solar-like star. Methods: We monitored ι Hor with XMM-Newton observations spanning almost two years. The spectra of each observation are fit with two-temperature coronal models to study the long-term variability of the star. Results: We find a cyclic behavior in X-rays very similar to the contemporaneous chromospheric cycle. The continuous chromospheric monitoring for more than three cycle lengths shows a trend toward decreasing amplitude, apparently modulated by a longer term trend. The second cycle is disrupted prior to reaching its maximum, followed by a brief episode of chaotic variability before the cyclic behavior resumes, only to be disrupted again after slightly more than one cycle. Conclusions: We confirm the presence of an activity cycle of ~1.6 yr in ι Hor both in X-rays and Ca ii H&K. It is likely subject to the modulation of a longer, not yet constrained second cycle. The 1.6 yr cycle is the shortest coronal one observed to date, and ι Hor represents the most active star for which a coronal activity cycle has been found. This cycle is probably representative of the first coronal cycles in the life of a solar-like star, at the age when life started on Earth. Table 2 is available in electronic form at http://www.aanda.org
Astronomy & Astrophysics, Volume 553, id.L6, 5 pp.May 2013 - X-shooter spectroscopy of FU Tauri A
Abstract
We have analyzed a broad-band optical and near-infrared spectrum of FU Tau A, which is a presumed young brown dwarf in the Taurus star forming region that has intrigued both theorists and observers by its overluminosity in the Hertzsprung-Russell (HR) diagram with respect to standard pre-main sequence evolutionary models. FU Tau A is brighter than any other Taurus member of the same or similar spectral type, and various phenomena (accretion, activity, binarity) have been put forth as possible explanations. The new data, obtained with the X-shooter spectrograph at the Very Large Telescope, include an unprecedented wealth of information on stellar parameters and simultaneously observed accretion and outflow indicators for FU Tau A. We present the first measurements of gravity (log g = 3.5 ± 0.5), radial velocity (RV = 22.5 ± 2.9 km s-1), rotational velocity (vsini = 20 ± 5 km s-1), and lithium equivalent width (EWLi = 430 ± 20 mÅ) for FU Tau A. From the rotational velocity and the published period we infer a disk inclination of i ≳ 50°. The lithium content is much lower than theoretically expected for such a young very low-mass object, adding another puzzling feature to this object
Astronomy & Astrophysics, Volume 551, id.A106, 12 pp.Mar 2013 - X-shooter spectroscopy of young stellar objects. II. Impact of chromospheric emission on accretion rate estimates
Abstract
Context. The lack of knowledge of photospheric parameters and the level of chromospheric activity in young low-mass pre-main sequence stars introduces uncertainties when measuring mass accretion rates in accreting (Class II) young stellar objects. A detailed investigation of the effect of chromospheric emission on the estimates of mass accretion rate in young low-mass stars is still missing. This can be undertaken using samples of young diskless (Class III) K and M-type stars. Aims: Our goal is to measure the chromospheric activity of Class III pre main sequence stars to determine its effect on the estimates of the accretion luminosity (Lacc) and mass accretion rate (Ṁacc) in young stellar objects with disks. Methods: Using VLT/X-shooter spectra, we analyzed a sample of 24 nonaccreting young stellar objects of spectral type between K5 and M9.5. We identified the main emission lines normally used as tracers of accretion in Class II objects, and we determined their fluxes in order to estimate the contribution of the chromospheric activity to the line luminosity. Results: We have used the relationships between line luminosity and accretion luminosity derived in the literature for Class II objects to evaluate the impact of chromospheric activity on the accretion rate measurements. We find that the typical chromospheric activity would bias the derived accretion luminosity by Lacc,noise< 10-3 L⊙, with a strong dependence on the Teff of the objects. The noise on Ṁacc depends on stellar mass and age, and the typical values of log(Ṁacc,noise) range between ~-9.2 to -11.6 M⊙/yr. Conclusions: Values of Lacc ≲10-3 L⊙ obtained in accreting low-mass pre main sequence stars through line luminosity should be treated with caution because the line emission may be dominated by the contribution of chromospheric activity. Based on observations collected in the programs 084.C-0269, 085.C-0238, 086.C-0173, 087.C-0244, 089.C-0143 at the European Organization for Astronomical Research in the Southern Hemisphere (Chile).Tables 6, 7, and Appendices are available in electronic form at http://www.aanda.org
Astronomy & Astrophysics, Volume 551, id.A107, 26 pp.Mar 2013
2012
- X-shooter spectroscopy of young stellar objects. I. Mass accretion rates of low-mass T Tauri stars in σ Orionis
Abstract
We present high-quality, medium-resolution X-shooter/VLT spectra in the range 300-2500 nm for a sample of 12 very low mass stars in the σ Orionis cluster. The sample includes eight stars with evidence of disks from Spitzer and four without disks, with masses ranging from 0.08 to 0.3 M⊙. The aim of this first paper is to investigate the reliability of the many accretion tracers currently used to measure the mass accretion rate in low-mass young stars and the accuracy of the correlations between these secondary tracers (mainly accretion line luminosities) found in the literature. We use our spectra to measure the accretion luminosity from the continuum excess emission in the UV and visual; the derived mass accretion rates range from 10-9 M⊙ yr-1 down to 5 × 10-11 M⊙ yr-1, allowing us to investigate the behavior of the accretion-driven emission lines in very low mass accretion rate regimes. We compute the luminosity of ten accretion-driven emission lines from the UV to the near-IR, which are all obtained simultaneously. In general, most of the secondary tracers correlate well with the accretion luminosity derived from the continuum excess emission. We recompute the relationships between the accretion luminosities and the line luminosities, and we confirm the validity of the correlations given in the literature, with the possible exception of Hα. Metallic lines, such as the CaII IR triplet or the Na I line at 589.3 nm, show a larger dispersion. When looking at individual objects, we find that the hydrogen recombination lines, from the UV to the near-IR, give good and consistent measurements of Lacc that often better agree than the uncertainties introduced by the adopted correlations. The average Lacc derived from several hydrogen lines, measured simultaneously, have a much reduced error. This suggests that some of the spread in the literature correlations may be due to the use of nonsimultaneous observations of lines and continuum. Three stars in our sample deviate from this behavior, and we discuss them individually. Based on observations collected at the European Southern Observatory, Chile. Program 084.C-0269(A), 086.C-0173(A).
Astronomy & Astrophysics, Volume 548, id.A56, 14 pp.Dec 2012 - LAMP: the long-term accretion monitoring programme of T Tauri stars in Chamaeleon I
Abstract
We present the results of a variability study of accreting young stellar objects in the Chameleon I star-forming region, based on ˜300 high-resolution optical spectra from the Fibre Large Area Multi-Element Spectrograph (FLAMES) at the European Southern Observatory (ESO) Very Large Telescope (VLT). 25 objects with spectral types from G2-M5.75 were observed 12 times over the course of 15 months. Using the emission lines Hα (6562.81 Å) and Ca II (8662.1 Å) as accretion indicators, we found 10 accreting and 15 non-accreting objects. We derived accretion rates for all accretors in the sample using the Hα equivalent width, Hα 10 per cent width and Ca II (8662.1 Å) equivalent width. We found that the Hα equivalent widths of accretors varied by ˜7-100 Å over the 15-month period. This corresponds to a mean amplitude of variations in the derived accretion rate of ˜0.37 dex. The amplitudes of variations in the derived accretion rate from Ca II equivalent width were ˜0.83 dex and those from Hα 10 per cent width were ˜1.11 dex. Based on the large amplitudes of variations in accretion rate derived from the Hα 10 per cent width with respect to the other diagnostics, we do not consider it to be a reliable accretion rate estimator. Assuming the variations in Hα and Ca II equivalent width accretion rates to be closer to the true value, these suggest that the spread that was found around the accretion rate to stellar-mass relation is not due to the variability of individual objects on time-scales of weeks to ˜1 year. From these variations, we can also infer that the accretion rates are stable within <0.37 dex over time-scales of less than 15 months. A major portion of the accretion variability was found to occur over periods shorter than the shortest time-scales in our observations, 8-25 days, which are comparable with the rotation periods of these young stellar objects. This could be an indication that what we are probing is spatial structure in the accretion flows and it also suggests that observations on time-scales of ˜a couple of weeks are sufficient to limit the total extent of accretion-rate variations in typical young stars. No episodic accretion was observed: all 10 accretors accreted continuously for the entire period of observations and, though they may have undetected low accretion rates, the non-accretors never showed any large changes in their emission that would imply a jump in accretion rate.
Monthly Notices of the Royal Astronomical Society, Volume 427, Issue 2, pp. 1344-1362.Dec 2012 - Spectroscopy of very low-mass stars and brown dwarfs in the Lambda Orionis star-forming region. II. Rotation, activity and other properties of spectroscopically confirmed members of Collinder 69
Abstract
Context. Most observational studies conducted so far point toward brown dwarfs sharing a similar formation mechanism as the one that is accepted for low-mass stars. However, larger databases and more systematic studies are needed before strong conclusions can be reached. Aims: In this second paper of a series devoted to studying the spectroscopic properties of the Lambda Orionis star-forming region members, we study accretion, activity and rotation for a wide set of spectroscopically confirmed members of the central star cluster Collinder 69 to assess analogies and/or differences between the brown-dwarf and stellar populations of this cluster. Moreover, we present comparisons with other star-forming regions of similar and different ages to address environmental effects on our conclusions. Methods: We studied prominent photospheric lines to derive rotational velocities and emission lines to distinguish between accretion processes and chromospheric activity. In addition, we include information about disk presence and X-ray emission. Results: We report very strong differences in the disk fractions of low-mass stars and brown dwarfs (~58%) when compared to higher mass stars (26-3+4%), with 0.6 M⊙ being the critical mass we find for this dichotomy. As a byproduct, we address the implications of the spatial distribution of disk and diskless members in the formation scenario of the cluster itself. We used the Hα emission to distinguish among accreting and non-accreting sources, finding that 38-7+8% of sources harboring disks undergo active accretion and that his percentage stays similar in the substellar regime. For these sources we have estimated accretion rates. Finally, regarding rotational velocities, we find a high dispersion in vsin(i) that is even higher among the diskless population. Based on the ESO observing programs 080.C-0592 and 078.C-0124; and observing programs from Calar Alto, Keck, Subaru, and Magellan.Appendices A and Table 6 are available in electronic form at http://www.aanda.org
Astronomy & Astrophysics, Volume 547, id.A80, 21 pp.Nov 2012 - Measuring the mass accretion rates of Herbig Ae/Be stars with X-shooter
Abstract
We present the results of our observations of eight magnetic Herbig Ae/Be stars obtained with the X-shooter spectrograph mounted on UT2 at the VLT. X-shooter provides a simultaneous, medium-resolution and high-sensitivity spectrum over the entire wavelength range from 300 to 2500 nm. We estimate the mass accretion rates (\dot{}}_acc) of the targets from 13 different spectral diagnostics using empiric calibrations derived previously for T Tauri-type stars and brown dwarfs. We have estimated the mass accretion rates of our targets, which range from 2×10-9 to 2×10-7 M⊙ yr-1. Furthermore, we have found accretion rate variability with amplitudes of 0.10-0.40 dex taking place on time scales from one day to tens of days. Additional future night-to-night observations need to be carried out to investigate the character of \dot{M}_acc variability in details. Our study shows that the majority of the calibration relations can be applied to Herbig Ae/Be stars, but several of them need to be re-calibrated on the basis of new spectral data for a larger number of Herbig Ae/Be stars. Based on observations obtained at the European Southern Observatory, Paranal, Chile (ESO programme 385.C-0131(A)).
Astronomische Nachrichten, Vol.333, Issue 7, p.594-612Aug 2012 - TYC 8380-1953-1: Discovery of an RS CVn Binary Through the XMM-Newton Slew Survey
Abstract
In this article we report the discovery of the chromospherically active (RS CVn type) binary TYC 8380-1953-1 through the XMM-Newton slew survey and present results of our optical and X-ray follow-up. With a flux limit of 6 × 10-13 ergs cm-2 s-1 in the soft band (0.2-2 keV), the XMM-Newton slew has a similar sensitivity to the ROSAT All Sky Survey, allowing interesting sources to be identified by their long-term variability. Two different types of stellar sources are detected in shallow X-ray surveys: young stars (both pre-main- and main-sequence stars) and chromospherically active binaries (BY Dra and RS CVn-type systems). The discovery of stars in such surveys and the study of their nature through optical follow-ups is valuable to determine their spatial distribution and scale height in the Galaxy. Our analysis shows that TYC 8380-1953-1 is a double-lined spectroscopic binary, with both components having similar spectral type (likely K0/2 + K3/5) and luminosity. With a typical coronal temperature for an RS CVn system (kT ~ 1.15 keV) and an X-ray luminosity in the 0.3-10 keV energy band higher than 4 × 1031 erg s-1, TYC 8380-1953-1 lies among the most X-ray luminous RS CVn binaries.
Publications of the Astronomical Society of Pacific, Volume 124, Issue 917, pp. 682-692 (2012).Jul 2012 - The ultracool dwarf DENIS-P J104814.7-395606. Chromospheres and coronae at the low-mass end of the main-sequence
Abstract
Context. Several diagnostics ranging from the radio to the X-ray band are suitable for investigating the magnetic activity of late-type stars. Empirical connections between the emission at different wavelengths place constraints on the nature and efficiency of the emission mechanism and the physical conditions in different atmospheric layers. The activity of ultracool dwarfs, at the low-mass end of the main-sequence, is poorly understood. Aims: We perform a multi-wavelength study of one of the nearest M9 dwarfs, DENIS-P J104814.7-395606 (4 pc), to examine its position within the group of magnetically active ultracool dwarfs, and, in general, advance our understanding of these objects by comparing them to early-M type dwarf stars and the Sun. Methods: We obtained an XMM-Newton observation of DENIS-P J104814.7-395606 and a broad-band spectrum from the ultraviolet to the near-infrared with X-Shooter. From this dataset, we derive the X-ray properties, stellar parameters, kinematics, and the emission-line spectrum tracing chromospheric activity. We integrate these data by compiling the activity parameters of ultracool dwarfs from the literature. Results: Our deep XMM-Newton observation provides the first X-ray detection of DENIS-P J104814.7-395606 (log Lx = 25.1), as well as the first measurement of its V band brightness (V = 17.35 mag). The flux-flux relations between X-ray and chromospheric activity indicators are here for the first time extended into the regime of the ultracool dwarfs. The approximate agreement of DENIS-P J104814.7-395606 and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests that the same heating mechanisms work in the atmospheres of ultracool dwarfs, albeit weaker as judged from their lower fluxes. The observed Balmer decrements of DENIS 1048-3956 are compatible with optically thick plasma in local thermal equilibrium (LTE) at low, nearly photospheric temperature or optically thin LTE plasma at 20 000 K. Describing the decrements with case B recombination requires different emitting regions for Hα and the higher Balmer lines. The high observed Hα/Hβ flux ratio is also poorly fitted by the optically thin models. We derive a similarly high value for the Hα/Hβ ratio of vB 10 and LHS 2065 and conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P J104814.7-395606 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Güdel relation between radio and X-ray luminosity of late-type stars is well-known to be violated by ultracool dwarfs. We speculate on the presence of two types of ultracool dwarfs with distinct radio and X-ray behaviors.
Astronomy & Astrophysics, Volume 537, id.A94, 13 pp.Jan 2012 - Magnetic activity and accretion on FU Tau A: clues from variability
Abstract
FU Tau A is a young very-low-mass object in the Taurus star-forming region which was previously found to have strong X-ray emission and to be anomalously bright for its spectral type. In this study we discuss these characteristics using new information from quasi-simultaneous photometric and spectroscopic monitoring. From photometric time series obtained with the 2.2-m telescope on Calar Alto, we measure a period of ˜4 d for FU Tau A, most likely the rotation period. The short-term variations over a few days are consistent with the rotational modulation of the flux by cool, magnetically induced spots. In contrast, the photometric variability on time-scales of weeks and years can only be explained by the presence of hotspots, presumably caused by accretion. The hotspot properties are thus variable on time-scales exceeding the rotation period, maybe due to long-term changes in the accretion rate or geometry. The new constraints from the analysis of the variability confirm that FU Tau A is affected by magnetically induced spots and excess luminosity from accretion. However, accretion is not sufficient to explain its anomalous position in the HR diagram. In addition, suppressed convection due to magnetic activity and/or an early evolutionary stage need to be invoked to fully account for the observed properties. These factors cause considerable problems in estimating the mass of FU Tau A and other objects in this mass/age regime, to the extent that it appears questionable if it is feasible to derive the initial mass function for young low-mass stars and brown dwarfs.
Monthly Notices of the Royal Astronomical Society, Volume 419, Issue 2, pp. 1271-1279.Jan 2012
2011
- Characterising the magnetic fields of the Herbig Ae/Be stars HD 97048, HD 150193, HD 176386, and MWC 480
Abstract
Context. Our knowledge of the presence and the role of magnetic fields in intermediate-mass pre-main-sequence stars remains very poor. Aims: We present the magnetic properties of four Herbig Ae/Be stars that have not been previously studied in detail. Methods: Our results for the three Herbig Ae/Be stars HD 97048, HD 150193, and HD 176386 are based on multi-epoch low-resolution spectra obtained in spectropolarimetric mode with FORS 2 mounted on the VLT. New high-resolution polarimetric spectra were obtained for MWC 480 with the SOFIN spectrograph installed at the Nordic Optical Telescope. We discuss 41 FORS 2 low-resolution observations of the Herbig Ae/Be stars HD 97048, HD 150193, and HD 176386 and determine their rotational periods. Using stellar fundamental parameters and the longitudinal magnetic field phase curves, we place constraints on the magnetic field geometry. Three high-resolution circularly polarised SOFIN spectra obtained for MWC 480 were measured using the moment technique where wavelength shifts between right- and left-hand side circularly polarised spectra are interpreted in terms of a longitudinal magnetic field ⟨Bz⟩. Results: Our search for periodicities resulted in P = 0.693 d for HD 97048, P = 1.317 d for HD 150193, and P = 0.899 d for HD 176386. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength Bd of several hundred Gauss. The longitudinal magnetic-field measurements of MWC 480 reveal the presence of a strong kG field, which was undetected in our previous low-resolution polarimetric observations with FORS 1. A weak magnetic field was detected in the circumstellar components of the Ca ii H&K lines and the Na i D lines, indicating a complex interaction between the stellar magnetic field and the circumstellar environment. Based on observations obtained at the European Southern Observatory (ESO programme 085.C-0137(A)) and SOFIN observations at the 2.56-m Nordic Optical Telescope on La Palma.
Astronomy & Astrophysics, Volume 536, id.A45, 8 pp.Dec 2011 - Spectroscopy of very low mass stars and brown dwarfs in the Lambda Orionis star forming region. I. Enlarging the census down to the planetary mass domain in Collinder 69
Abstract
Context. Whilst there is a generally accepted evolutionary scheme for the formation of low-mass stars, the analogous processes when moving down in mass to the brown dwarf regime are not yet well understood. Aims: In this first paper, we try to compile the most complete and unbiased spectroscopically confirmed census of the population of Collinder 69, the central cluster of the Lambda Orionis star forming region, as a first step in addressing the question of how brown dwarfs and planetary mass objects form. Methods: We study age dependent features in optical and near-infrared spectra of candidate members to the cluster (such as alkali lines and accretion-associated indicators). In addition, we complement that study with the analysis of other youth indicators, such as X-ray emission or mid-infrared excess. Results: We confirm the membership to Collinder 69 of ~90 photometric candidate members. As a byproduct, we determine a temperature scale for young M, very low-mass stars, and brown dwarfs. We assemble one of the most complete initial mass functions from 0.016 to 20 M⊙. Finally, we study the implications of the spatial distribution of the confirmed members for the proposed mechanisms of brown dwarf formation. Appendix A is available in electronic form at http://www.aanda.orgTable 6 is also 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/536/A63
Astronomy & Astrophysics, Volume 536, id.A63, 23 pp.Dec 2011 - Recent studies of magnetic fields in Herbig Ae/Be stars
Abstract
New determinations of the mean longitudinal magnetic field for several Herbig Ae/Be stars are presented. The longitudinal magnetic field measurements of MWC 480 reveal the presence of a strong kG field, which was undetected in our previous low-resolution polarimetric observations with FORS 1. The magnetic field geometries of Herbig Ae/Be stars studied with spectropolarimetric time series can likely be described by centred dipoles with polar magnetic field strengths of several hundred Gauss. A number of Herbig Ae/Be stars with detected magnetic fields were recently observed with X-shooter in the visible and near-IR as well as with the high-resolution near-IR spectrograph CRIRES. These observations are of great importance to understand the relation between the magnetic field topology and the physics of accretion flow and accretion disk gas emission. Based on observations obtained at the European Southern Observatory, Paranal, Chile (ESO programmes 085.C-0137(A), 085.C-0137(A), 087.C-0124(A)) and SOFIN observations at the 2.56-m Nordic Optical Telescope on La Palma.}
Astronomische Nachrichten, Vol.332, Issue 9/10, p.1022Dec 2011 - The First X-shooter Observations of Jets from Young Stars
Abstract
We present the first pilot study of jets from young stars conducted with X-shooter, on the ESO/Very Large Telescope. As it offers simultaneous, high-quality spectra in the range 300-2500 nm, X-shooter is uniquely important for spectral diagnostics in jet studies. We chose to probe the accretion/ejection mechanisms at low stellar masses examining two targets with well-resolved continuous jets lying on the plane of the sky: ESO-HA 574 in Chameleon I and Par-Lup3-4 in Lupus III. The mass of the latter is close to the sub-stellar boundary (M sstarf = 0.13 M sun). A large number of emission lines probing regions of different excitation are identified, position-velocity diagrams are presented, and mass outflow/accretion rates are estimated. Comparison between the two objects is striking. ESO-HA 574 is a weakly accreting star for which we estimate a mass accretion rate of log (\dot{M}_{acc}) = -10.8 +/- 0.5 (in M sun yr-1), yet it drives a powerful jet with \dot{M}_{out} ~ 1.5-2.7 × 10-9 M sun yr-1. These values can be reconciled with a magneto-centrifugal jet acceleration mechanism assuming that the presence of the edge-on disk severely depresses the luminosity of the accretion tracers. In comparison, Par-Lup3-4, with stronger mass accretion (log (\dot{M}_{acc}) = -9.1 +/- 0.4 M sun yr-1), drives a low-excitation jet with about \dot{M}_{out} ~ 3.2 × 10-10 M sun yr-1 in both lobes. Despite the low stellar mass, \dot{M}_{out}/\dot{M}_{acc} for Par-Lup3-4 is at the upper limit of the range usually measured for young objects, but still compatible with a steady magneto-centrifugal wind scenario if all uncertainties are considered. Based on Observations collected with X-shooter at the Very Large Telescope on Cerro Paranal (Chile), operated by the European Southern Observatory (ESO). Program ID: 085.C-0238(A).
The Astrophysical Journal Letters, Volume 737, Issue 2, article id. L26, 7 pp. (2011).Aug 2011 - Search of X-ray emission from roAp stars: the case of γ Equulei
Abstract
Context. Rapidly oscillating Ap (roAp) stars represent a subclass of magnetic, chemically peculiar stars. The explanation for their pulsations includes suppressed convection due to the strong magnetic field. These stars rotate slowly such that a solar-like dynamo and ensuing magnetic activity is unlikely to be present. On the other hand, magnetic activity could provide the particle acceleration suspected to be responsible for the presence of short-lived radionuclides on some roAp stars. Aims: The detection of X-ray emission from Ap stars can be an indicator for the presence of magnetic activity and dynamo action, provided different origins for the emission, such as wind shocks and close late-type companions, can be excluded. Here we report on results for γ Equ, the only roAp star for which an X-ray detection is reported in ROSAT catalogs. Methods: We use high resolution imaging in X-rays with Chandra and in the near-infrared with NACO/VLT that allow us to spatially resolve companions down to ≤ 1
Astronomy & Astrophysics, Volume 529, id.A29, 5 pp.May 2011 - Extremely metal-poor stars in SDSS fields
Abstract
Some insight on the first generation of stars can be obtained from the chemical composition of their direct descendants, extremely metal-poor stars (EMP), with metallicity less than or equal to 1/1000 of the solar metallicity. Such stars are exceedingly rare, the most successful surveys, for this purpose, have so far provided only about 100 stars with 1/1 000 the solar metallicity and 4 stars with about 1/10 000 of the solar metallicity. The Sloan Digital Sky Survey has the potential to provide a large number of candidates of extremely low metallicity. X-shooter has the unique capability of performing the necessary follow-up spectroscopy providing accurate metallicities and abundance ratios for several elements (Mg, Al, Ca, Ti, Cr, Sr, ...) for EMP candidates. We here report on the results for the first two stars observed in the course of our Franco-Italian X-shooter GTO. The two stars were targeted to be of metallicity around -3.0, the analysis of the X-shooter spectra showed them to be of metallicity around -2.0, but with a low α to iron ratio, which explains the underestimate of the metallicity from the SDSS spectra. The efficiency of X-shooter allows an in situ study of the outer halo, for the two stars studied here we estimate distances of 3.9 and 9.1 kpc, these are likely the most distant dwarf stars studied in detail to date. Based on spectra obtained with X-shooter at the 8.2-m Kueyen ESO telescope, GTO programmes 085.D-0194 and 086.D.0094.
Astronomische Nachrichten, Vol.332, Issue 3, p.251-257Mar 2011 - XMM-Newton investigations of the Lambda Orionis star-forming region (XILO). I. The young cluster Collinder 69
Abstract
Context. This is the first paper of a series devoted to the Lambda Orionis star-forming region, Orion
Astronomy and Astrophysics, Volume 526, id.A21, 34 pp.Feb 2011
2010
- The nature of the recent extreme outburst of the Herbig Be/FU Orionis binary Z Canis Majoris
Abstract
Context. Z CMa is a binary system which consists of two young stars: a Herbig AeBe component Z CMa NW embedded in a dust cocoon and a less massive component Z CMa SE, which is classified as a FU Orionis type star. Associated to the binary system is a giant parsec-size jet. Past spectropolarimetric observations showed that the position angle of the linear optical polarization is perpendicular to the jet axis, indicating that the visual light escapes the cocoon via cavities aligned with the jet axis and is then scattered back into the line of sight of the observer. Recently the system showed the largest outburst reported during the almost 90 years of available observations. Aims: We present new spectrophotometric and spectropolarimetric data obtained in 2008 during the recent outburst phase. Methods: The data obtained in the visual spectral range at medium spectral resolution were used to study the geometry of the system from the linear polarization spectra as well as its magnetic field from the circular polarization spectra. Results: During the recent outburst we detected that the Z CMa system is polarized by 2.6% in the continuum and emission line spectrum, with a position angle still perpendicular to the jet. From the high level of polarization we concluded that the outburst is associated with the dust-embedded Herbig AeBe NW component. The deep absorption components of the Balmer lines in the velocity frame which extend from zero velocity and reach a wind velocity of 700 km s-1, together with the absence of a red-shifted broad emission at similar velocities, indicate a bi-polar wind. We did not detect a significant mean longitudinal magnetic field during the outburst, but in the data obtained in 2004 we detected the possible presence of a rather strong magnetic field of the order of 1 kG. However, we critically review the applied method of magnetic field measurements in the presence of a strong stellar wind. The main result of our studies is that the bolometric luminosity of Z CMa remained surprisingly constant during the recent outburst. We conclude that either the geometry of the cavity through which the light escapes from the cocoon has opened a new path or that the screen of dust, which reflects the light toward the observer became more efficient, causing the observed increase of the visual brightness by about 2fm5. Based on observations made with ESO Telescopes at the Paranal Observatories under programme IDs: 074.C-0442, 081.C-0410, and 282.C-5041.Appendix A, Tables 1 and 2, and Fig. 6 are only available at http://www.aanda.org
Astronomy and Astrophysics, Volume 509, id.L7, 5 pp.Jan 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 - A Chandra and Spitzer census of the young star cluster in the reflection nebula NGC 7129
Abstract
Context: The reflection nebula NGC 7129 has long been known to be a site of recent star formation as shown, e.g., by the presence of deeply embedded protostars and HH objects. However, studies of the stellar population produced in the star formation process have remained rudimentary. A major step forward has been made with recent Spitzer imaging of the region. Aims: This study represents the next step towards a systematic assessment of the pre-main sequence population in NGC 7129. Completeness of the pre-main sequence sample is necessary for studying key features that allow the star-forming process to be understood, such as disk evolution, dynamical evolution, and mass function. At a presumed age of 3 Myr, NGC 7129 is in the critical range where disks around young stars disappear. Methods: We make use of X-ray and IR imaging observations to identify the pre-main sequence stars in NGC 7129. We define a sample of young stellar objects based on color-color diagrams composed from IR photometry between 1.6 and 8 μm, from 2 MASS and Spitzer, and based on X-ray detected sources from a Chandra observation. Results: This sample is composed of 26 Class II and 25 Class III candidates. It has been selected from infrared sources in the Chandra field (287 objects with photometry in all four Spitzer/IRAC bands, 811 objects with near-IR photometry) and the 59 X-ray sources detected with Chandra. The sample is estimated to be complete down to 0.5 {M_⊙}. The most restricted and least biased subsample of pre-main sequence stars is composed of lightly absorbed (AV < 5 mag) stars in the cluster core. This sample comprises 7 Class II and 14 Class III sources and has a disk fraction of 33^+24-19% and a median X-ray luminosity of log{L_x} [erg/s] = 30.3. Conclusions: Despite the various uncertainties related to the sample selection, absorption, mass distribution, distance, and, consequently in the computation of disk fraction and X-ray luminosities, the data yield consistent results. In particular, we confirm the age of ˜ 3 Myr for the NGC 7129 cluster. The derived disk fraction is similar to that of σ Orionis, smaller than for Cha I (˜ 2 Myr), and larger than for Upper Sco (5 Myr). The X-ray luminosity function is similar to that of NGC 2264 (2 Myr) but fainter than for the Orion Nebula Cluster (1 Myr). The pre-main sequence census should be further refined and extended with optical photometric and spectroscopic searches for cluster members. Tables 2-6 are only available in electronic form at http://www.aanda.org
Astronomy and Astrophysics, Volume 507, Issue 1, 2009, pp.227-240Nov 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 - Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks
Abstract
Context: Recently, evidence for the presence of weak magnetic fields in Herbig Ae/Be stars has been found in several studies. Aims: We seek to expand the sample of intermediate-mass pre-main sequence stars with circular polarization data to measure their magnetic fields, and to determine whether magnetic field properties in these stars are correlated with mass-accretion rate, disk inclination, companions, silicates, PAHs, or show a correlation with age and X-ray emission as expected for the decay of a remnant dynamo. Methods: Spectropolarimetric observations of 21 Herbig Ae/Be stars and six debris disk stars have been obtained at the European Southern Observatory with FORS 1 mounted on the 8 m Kueyen telescope of the VLT. With the GRISM 600B in the wavelength range 3250-6215 Å we were able to cover all hydrogen Balmer lines from Hβ to the Balmer jump. In all observations a slit width of 0.4 arcsec was used to obtain a spectral resolving power of R ≈ 2000. Results: Among the 21 Herbig Ae/Be stars studied, new detections of a magnetic field were achieved in six stars. For three Herbig Ae/Be stars, we confirm previous magnetic field detections. The largest longitudinal magnetic field, = -454±42 G, was detected in the Herbig Ae/Be star HD 101412 using hydrogen lines. No field detection at a significance level of 3σ was achieved in stars with debris disks. Our study does not indicate any correlation of the strength of the longitudinal magnetic field with disk orientation, disk geometry, or the presence of a companion. We also do not see any simple dependence on the mass-accretion rate. However, it is likely that the range of observed field values qualitatively supports the expectations from magnetospheric accretion models giving support for dipole-like field geometries. Both the magnetic field strength and the X-ray emission show hints of a decline with age in the range of ~2-14 Myr probed by our sample, supporting a dynamo mechanism that decays with age. However, our study of rotation does not show any obvious trend of the strength of the longitudinal magnetic field with rotation period. Furthermore, the stars seem to obey the universal power-law relation between magnetic flux and X-ray luminosity established for the Sun and main-sequence active dwarf stars. Based on observations obtained at the European Southern Observatory, Paranal, Chile (ESO programmes 077.C-0521(A) and 081.C-0410(A)).
Astronomy and Astrophysics, Volume 502, Issue 1, 2009, pp.283-301Jul 2009 - New X-ray detections of Herbig stars
Abstract
Context: The interpretation of X-ray detections from Herbig Ae/Be stars is disputed as it is not clear whether these intermediate-mass pre-main sequence stars are able to drive a dynamo and ensuing phenomena of magnetic activity. Alternative X-ray production mechanisms, related to stellar winds, star-disk magnetospheres, or unresolved late-type T Tauri star companions have been proposed. Aims: The companion hypothesis can be tested by resolving Herbig stars in X-rays from their known visual secondaries. Furthermore, their global X-ray properties (such as detection rate, luminosity, temperature, variability) may give clues to the emission mechanism by comparison to other types of stars, e.g. similar-age but lower-mass T Tauri stars, similar-mass but more evolved main-sequence A- and B-type stars, and with respect to model predictions. Methods: In a series of papers we have been investigating high-resolution X-ray Chandra images of Herbig Ae/Be and main-sequence B-type stars where known close visual companions are spatially separated from the primaries. Results: Here we report on six as yet unpublished Chandra exposures from our X-ray survey of Herbig stars. The target list comprises six Herbig stars with known cool companions, and three other A/B-type stars that are serendipitously in the Chandra field-of-view. In this sample we record a detection rate of 100%; i.e. all A/B-type stars display X-ray emission at levels of log(L_x/L_bol) ~ -5...-7. The analysis of hardness ratios confirms that HAeBes have hotter and/or more absorbed X-ray emitting plasma than more evolved B-type stars. Conclusions: Radiative winds are ruled out as an exclusive emission mechanism on the basis of the high X-ray temperatures. Confirming earlier results, the X-ray properties of Herbig Ae/Be stars are not vastly different from those of their late-type companion stars (if such are known). The diagnostics provided by the presently available data leave it open whether the hard X-ray emission of Herbig stars is due to young age or indicative of further coronally active low-mass companion stars. In the latter case, our detection statistics imply a high fraction of higher order multiple systems among Herbig stars.
Astronomy and Astrophysics, Volume 493, Issue 3, 2009, pp.1109-1119Jan 2009
2007
- Emission Line Variability of the Accreting Young Brown Dwarf 2MASSW J1207334-393254: From Hours to Years
Abstract
We have obtained a series of high-resolution optical spectra for 2MASSW J1207334-393254 (2M 1207). Two consecutive observing nights at the ESO Very Large Telescope with the Ultraviolet and Visual Echelle Spectrograph yielded a time series with a resolution of ~12 minutes. Additional high-resolution optical spectra were obtained months apart at the Magellan Clay telescope using the Magellan Inamori Kyocera Echelle (MIKE) instrument. Combined with previously published results, these data allow us to investigate changes in the emission line spectrum of 2M 1207 on timescales of hours to years. Most of the emission line profiles of 2M 1207 are broad, in particular that of Hα, indicating that the dominant fraction of the emission must be attributed to disk accretion rather than to magnetic activity. From the Hα 10% width, we deduce a relatively stable accretion rate between 10-10.1 and 10-9.8 Msolar yr-1 for two nights of consecutive observations. Therefore, either the accretion stream is nearly homogeneous over (sub)stellar longitude, or the system is seen face-on. Small but significant variations are evident throughout our near-continuous observation, and they reach a maximum after ~8 hr, roughly the timescale on which maximum variability is expected across the rotation cycle. Together with past measurements, we confirm that the accretion rate of 2M 1207 varies by more than 1 order of magnitude on timescales of months to years. Such variable mass accretion yields a plausible explanation for the observed spread in the M˙-M diagram. The magnetic field required to drive the funnel flow is on the order of a few hundred G. Despite the obvious presence of a magnetic field, neither radio nor X-ray emission has been reported for 2M 1207. It is possible that strong accretion suppresses magnetic activity in brown dwarfs, similar to the findings for higher mass T Tauri stars.
The Astrophysical Journal, Volume 671, Issue 1, pp. 842-852.Dec 2007 - Establishing the nature of companion candidates to X-ray-emitting late B-type stars
Abstract
The most favoured interpretation for the detection of X-ray emission from late B-type stars is that these stars have a yet undiscovered late-type companion (or an unbound nearby late-type star) that produces the X-rays. Several faint infrared objects at (sub)arcsecond separation from B-type stars have been uncovered in our earlier adaptive optics imaging observations, and some of them have been followed up with the high spatial resolution of the Chandra X-ray observatory, pinpointing the X-ray emitter. However, firm conclusions on their nature require a search for spectroscopic signatures of youth. Here we report on our recent ISAAC observations carried out in low-resolution spectroscopic mode. Equivalent widths have been used to obtain information on spectral types of the companions. All eight X-ray-emitting systems with late B-type primaries studied contain dwarf-like companions with spectral types later than A7. The only system in the sample where the companion turns out to be of early spectral type is not an X-ray source. These results are consistent with the assumption that the observed X-ray emission from late B-type stars is produced by an active pre-main-sequence companion star. Based on observations obtained at the European Southern Observatory, Paranal, Chile [ESO programmes 074.D-0374(A) and 075.C-0522(A)]. E-mail: shubrig@eso.org
Monthly Notices of the Royal Astronomical Society, Volume 381, Issue 4, pp. 1569-1574.Nov 2007 - Evolution of Brown Dwarf Disks: A Spitzer Survey in Upper Scorpius
Abstract
We have carried out a Spitzer survey for brown dwarf disks in the ~5 Myr old Upper Scorpius (UpSco) star-forming region, using IRS spectroscopy from 8 to 12 μm and MIPS photometry at 24 μm. Our sample consists of 35 confirmed very low mass members of UpSco. Thirteen objects in this sample show clear excess flux at 24 μm, explained by dust emission from a circum-substellar disk. The spectral energy distributions (SEDs) of the remaining objects are consistent with pure photospheric emission. Objects without excess emission have either no disks or disks with inner opacity holes of at least ~5 AU radii. Our disk frequency of 37%+/-9% is higher than what has been derived previously for K0-M5 stars in the same region (1.8 σ confidence level), suggesting a mass-dependent disk lifetime in UpSco. The clear distinction between objects with and without disks, as well as the lack of transition objects, shows that disk dissipation inside 5 AU occurs rapidly, probably on timescales of <~105 yr. For the objects with disks, most SEDs are uniformly flat, with flux levels of a few mJy, well modeled as emission from dusty disks affected by dust settling to the midplane. The silicate feature around 10 μm is either absent or weak in our SEDs, arguing for a lack of hot, small dust grains. Compared with younger objects in Taurus, brown dwarf disks in UpSco show less flaring. These results clearly demonstrate that we see disks in an advanced evolutionary state. Dust settling and grain growth are ubiquituous in circum-substellar disks at ages of 5 Myr, arguing for planet-forming processes in brown dwarf disks.
The Astrophysical Journal, Volume 660, Issue 2, pp. 1517-1531.May 2007
2005
- XMM-Newton spectroscopy of the metal depleted T Tauri star TWA 5
Abstract
We present results of X-ray spectroscopy for TWA 5, a member of the young TW Hydrae association, observed with XMM-Newton. TWA 5 is a multiple system which shows Hα emission, a signature typical of classical T Tauri stars, but no infrared excess. From this analysis of the RGS and EPIC spectra, we have derived the emission measure distribution vs. temperature of the X-ray emitting plasma, its abundances, and the electron density. The characteristic temperature and density of the plasma suggest a corona similar to that of weak-line T Tauri stars and active late-type main sequence stars. TWA 5 also shows low iron abundance (~0.1 times the solar photospheric one) and a pattern of increasing abundances for elements with increasing first ionization potential reminiscent of the inverse FIP effect observed in highly active stars. The especially high ratio Ne/Fe˜10 is similar to that of the classical T Tauri star TW Hya, where the accreting material has been held responsible for the X-ray emission. We discuss the possible role of an accretion process in this scenario. Since all T Tauri stars in the TW Hydrae association studied so far have very high Ne/Fe ratios, we also propose that environmental conditions may cause this effect.
Astronomy and Astrophysics, Volume 439, Issue 3, September I 2005, pp.1149-1158Sep 2005 - Discovery of Extremely Embedded X-Ray Sources in the R Coronae Australis Star-forming Core
Abstract
With the XMM-Newton and Chandra observatories, we detected two extremely embedded X-ray sources in the R Corona Australis (R CrA) star-forming core, near IRS 7. These sources, designated as XE and XW, have X-ray absorption columns of ~3×1023 cm-2 equivalent to AV~180 mag. They are associated with the VLA centimeter radio sources 10E and 10W, respectively; XW is the counterpart of the near-infrared source IRS 7, whereas XE has no K-band counterpart above 19.4 mag. This indicates that XE is younger than typical Class I protostars, probably a Class 0 protostar, or in an intermediate phase between Class 0 and Class I. The X-ray luminosity of XE varied between 29<logLX<31.2 ergs s-1 on timescales of 3-30 months; XE also showed a monotonic increase in X-ray brightness by a factor of 2 in 30 ks during an XMM-Newton observation. The XMM-Newton spectra indicate emission from a hot plasma with kT~3-4 keV and also show fluorescent emission from cold iron. Although the X-ray spectrum from XE is similar to flare spectra from Class I protostars in luminosity and temperature, the light curve does not resemble the light curves of magnetically generated X-ray flares, because the variability timescale of XE is too long and variations in X-ray count rate were not accompanied by variations in spectral hardness. The short-term variation of XE may be caused by the partial blocking of the X-ray plasma, while the month-long flux enhancement may be driven by mass accretion.
The Astrophysical Journal, Volume 623, Issue 1, pp. 291-301.Apr 2005 - X-rays from accretion shocks in T Tauri stars: The case of BP Tau
Abstract
We present an XMM-Newton observation of the classical T Tauri star BP Tau. In the XMM-Newton RGS spectrum the O vii triplet is clearly detected with a very weak forbidden line indicating high plasma densities and/or a high UV flux environment. At the same time concurrent UV data point to a small hot spot filling factor suggesting an accretion funnel shock as the site of the X-ray and UV emission. Together with the X-ray data on TW Hya these new observations suggest such funnels to be a general feature in classical T Tauri stars.
Astronomy and Astrophysics, Volume 432, Issue 2, March III 2005, pp.L35-L38Mar 2005
2004
- The weak-line T Tauri star V410 Tau. II. A flaring star
Abstract
We show that V410 Tau, a weak-line T Tauri star, is a flaring star. This result comes from an intensive, coordinated monitoring campaign carried out in November 2001 at visible and X-ray wavelength ranges. It is confirmed by previous, isolated observations found in the literature. Flares tend to occur mainly around the star
Astronomy and Astrophysics, v.427, p.263-278 (2004)Nov 2004 - Quiescent X-Ray Emission from an Evolved Brown Dwarf?
Abstract
I report on the X-ray detection of Gl 569Bab. During a 25 ks Chandra observation, the binary brown dwarf is for the first time spatially separated in X-rays from the flare star primary Gl 569A. Companionship to Gl 569A constrains the age of the brown dwarf pair to ~300-800 Myr. The observation presented here is only the second X-ray detection of an evolved brown dwarf. About half of the observing time is dominated by a large flare on Gl 569Bab; the remainder is characterized by weak and nonvariable emission just above the detection limit. This emission-if not related to the afterglow of the flare-represents the first detection of a quiescent corona on a brown dwarf, representing an important piece in the puzzle of dynamos in the substellar regime.
The Astrophysical Journal, Volume 615, Issue 2, pp. L153-L156.Nov 2004 - X-ray emission from a metal depleted accretion shock onto the classical T Tauri star TW Hya
Abstract
We present the X-ray spectrum of TW Hya observed at high and intermediate spectral resolution with the Reflection Grating Spectrometer (RGS) and the European Photon Imaging Camera (EPIC) onboard the XMM-Newton satellite. TW Hya is the first classical T Tauri star for which simultaneous X-ray data with both high spectral resolution and high sensitivity were obtained, thus allowing to probe the X-ray emission properties of stars in the early pre-main sequence phase. Despite TW Hya
Astronomy and Astrophysics, v.418, p.687-697 (2004)May 2004
2003
- The weak-line T Tauri star V410 Tau. I. A multi-wavelength study of variability
Abstract
We present the results of an intensive coordinated monitoring campaign in the optical and X-ray wavelength ranges of the low-mass, pre-main sequence star V410 Tau carried out in November 2001. The aim of this project was to study the relation between various indicators for magnetic activity that probe different emitting regions and would allow us to obtain clues on the interplay of the different atmospheric layers: optical photometric star spot (rotation) cycle, chromospheric Hα emission, and coronal X-rays. Our optical photometric monitoring has allowed us to measure the time of the minimum of the lightcurve with high precision. Joining the result with previous data we provide a new estimate for the dominant periodicity of V410 Tau (1.871970 +/- 0.000010 d). This updated value removes systematic offsets of the time of minimum observed in data taken over the last decade. The recurrence of the minimum in the optical lightcurve over such a long timescale emphasizes the extraordinary stability of the largest spot. This is confirmed by radial velocity measurements: data from 1993 and 2001 fit almost exactly onto each other when folded with the new period. The combination of the new data from November 2001 with published measurements taken during the last decade allows us to examine long-term changes in the mean light level of the photometry of V410 Tau. A variation on the timescale of 5.4 yr is suggested. Assuming that this behavior is truly cyclic V410 Tau is the first pre-main sequence star on which an activity cycle is detected. Two X-ray pointings were carried out with the Chandra satellite simultaneously with the optical observations, and centered near the maximum and minimum levels of the optical lightcurve. A relation of their different count levels to the rotation period of the dominating spot is not confirmed by a third Chandra observation carried out some months later, during another minimum of the 1.87 d cycle. Similarly we find no indications for a correlation of the Hα emission with the spots
Astronomy and Astrophysics, v.411, p.517-531 (2003)Dec 2003 - A Disk Census for Young Brown Dwarfs
Abstract
Recent surveys have identified substellar objects down to planetary masses in nearby star-forming regions. Reliable determination of the disk frequency in young brown dwarfs is of paramount importance to understanding their origin. Here we report the results of a systematic study of infrared L
The Astronomical Journal, Volume 126, Issue 3, pp. 1515-1521.Sep 2003 - Late B-type stars and their candidate companions resolved with Chandra
Abstract
We present the first results from a series of Chandra observations carried out with the aim to examine the origin of X-ray emission in main-sequence late B-type stars. X-ray detections of late-B and early A-type stars have remained a mystery as none of the two major theories for stellar X-ray emission applies in this spectral range: while O- and early B-type stars drive strong winds that are subject to instabilities, late-type stars produce X-rays as a result of magnetic dynamo action. Since any dynamo works only in the presence of a convective zone, early-type stars are not magnetically active. We use high spatial resolution X-ray observations to enlighten the prevalent speculation that previously unknown late-type or low-mass companion stars are the sites of the X-ray emission, instead of the B-type primaries. Here we present the results for HD 1685, HD 113703, HD 123445, HD 133880, and HD 169978. Adaptive optics observations have recently revealed at least one faint object near each of these B-type stars (at separation of 1-6''). Four of the new infrared objects show infrared colors and magnitudes typical for low-mass pre-main sequence stars, and are likely true companions to the ~ 10-50 Myr old B-type stars. These multiple systems are now resolved for the first time in X-ray light. We uncover that four of the new companions are X-ray emitters, and the fifth one is likely to be a weak X-ray source below the detection limit. Three of the B-type primaries are X-ray dark down to the detection limit of Lx ~ 1028 erg/s. But we do detect X-ray emission from the position of HD 1685 A and HD 169978 A. The latter one indeed is a spectroscopic binary. The characteristics of all X-ray sources are compatible with those of typical young late-type stars: hard X-ray spectrum (kT > 0.5 keV) and high X-ray luminosity (log Lx ~ 29...30 erg/s). Spectroscopic observations in the infrared will solve the question whether the one remaining X-ray detected B-star in our sample, HD 1685 A, also has an even closer companion or whether this is an intrinsic X-ray emitter. This work made use of observations obtained at the European Southern Observatory, La Silla, Chile (ESO program No. 67.C-0261(B)).
Astronomy and Astrophysics, v.407, p.1067-1078 (2003)Sep 2003 - Castor A and Castor B resolved in a simultaneous Chandra and XMM-Newton observation
Abstract
We present a simultaneous Chandra and XMM-Newton observation of the Castor sextett, focusing on Castor A and Castor B, two spectroscopic binaries with early-type primaries. Our study represents the first unambiguous X-ray detection of all three visual components in the Castor sextett making use of the unprecedented spatial resolution of Chandra. Of the present day X-ray instruments only Chandra can isolate the X-ray lightcurves and spectra of Castor A and B (angular separation ~ 4
Astronomy and Astrophysics, v.402, p.719-728 (2003)May 2003
2002
- X-ray emission near the substellar limit: The sigma Orionis and Taurus star forming regions
Abstract
We have carried out an extensive search for X-ray emission from young, very low-mass objects near and beyond the substellar limit, making use of archived ROSAT PSPC and HRI observations pointed at Brown Dwarfs and Brown Dwarf candidates in the young sigma Orionis and Taurus-Auriga associations. In sigma Ori we identify three Brown Dwarf candidates with X-ray sources; in Taurus-Auriga we add one further X-ray detection of a Brown Dwarf to the list published earlier. We combine this data with all previously X-ray detected Brown Dwarfs and Brown Dwarf candidates in young stellar associations and star forming regions to perform a study of stellar activity parameters on the as yet largest sample of young, very low mass objects. A similar relation between X-ray and bolometric luminosity, and Hα emission, respectively, as is known for T Tauri stars seems to hold for young objects down to the substellar limit, too. No signs for a change in X-ray activity are found on the transition to substellar masses.
Astronomy and Astrophysics, v.391, p.1025-1032 (2002)Sep 2002 - Simultaneous X-ray spectroscopy of YY Gem with Chandra and XMM-Newton
Abstract
We report on a detailed study of the X-ray spectrum of the nearby eclipsing spectroscopic binary YY Gem. Observations were obtained simultaneously with both large X-ray observatories, XMM-Newton and Chandra. We compare the high-resolution spectra acquired with the Reflection Grating Spectrometer onboard XMM-Newton and with the Low Energy Transmission Grating Spectrometer onboard Chandra, and evidence in direct comparison the good performance of both instruments in terms of wavelength and flux calibration. The strongest lines in the X-ray spectrum of YY Gem are from oxygen. Oxygen line ratios indicate the presence of a low-temperature component (1-4 MK) with density n_e 107 K are relevant in the corona of YY Gem although not as dominant as the lower temperatures represented by the strongest lines in the high-resolution spectrum. Magnetic loops with length on the order of 109 cm, i.e., about 5% of the radius of each star, are inferred from a comparison with a one-dimensional hydrodynamic model. This suggests that the flares did not erupt in the (presumably more extended) inter-binary magnetosphere but are related to one of the components of the binary.
Astronomy and Astrophysics, v.392, p.585-598 (2002)Sep 2002
2001
- X-ray emission from young stars in Taurus-Auriga-Perseus: Luminosity functions and the rotation – activity – age – relation
Abstract
We report on a systematic search for X-ray emission from pre-main sequence and young main sequence stars in the Taurus-Auriga-Perseus region. Our stellar sample consists of all T Tauri stars from the Taurus-Auriga region, and all late-type stars from the Pleiades and Hyades clusters which have been observed by the ROSAT PSPC in pointed observations. We present the X-ray parameters for all observed stars in tables. Next to the basic results of the data analysis (such as count rates, exposure time, and off-axis angle) we give X-ray luminosities and hardness ratios for all detected stars. Upper limits are given for non-detections. Detection rates for different spectral types are compiled. We use these results to study the connection between coronal X-ray activity and stellar parameters for different subgroups of our sample. In particular we compile X-ray luminosity functions (XLF), and discuss the relations between X-ray emission and spectral type, age, and rotation, which have been disputed extensively in the past. Here, we study these questions with the largest sample so far. The XLF for classical and weak-line T Tauri stars are different, with weak-lines being the stronger X-ray emitters. Proceeding towards the main-sequence (Pleiades, Hyades) the X-ray luminosity declines for all spectral types examined (G, K, and M stars). Within an age group Lx decreases towards later spectral types, while Lx/Lbol remains constant or even increases, reflecting the opposed influence of stellar radius, i.e. emitting area, and convection zone depth. For a given spectral type the fastest rotators show the highest X-ray luminosity. Rotation rate and X-ray emission are clearly correlated for all groups of stars with power law indices for log(Lx/Lbol) versus lg{Prot} of ~ -0.7 to -1.5. The study of XLF for binary stars shows that the known unresolved secondaries likely contribute a significant amount to the X-ray emission. Tables 2 to 7 are only available in electronic form at http://www.edpsciences.org
Astronomy and Astrophysics, v.377, p.538-556 (2001)Oct 2001 - X-ray emission of multiple T Tauri stars in Taurus
Abstract
We present a study of X-ray emission of known multiple T Tauri stars (TTS) in Taurus based on ROSAT observations. We used the ROSAT All-Sky Survey (RASS) detection rates of single classical (cTTS) and weak-line TTS (wTTS) to investigate statistically the TTS nature (classical or weak-line) of the components in multiple TTS, which are too close for spatially resolved spectroscopy so far. Because single wTTS show a higher RASS detection rate than single cTTS, the different binary TTS (cTTS-cTTS, cTTS-wTTS, wTTS-wTTS) should also have different detection rates. We find that the observed RASS detection rates of binary wTTS, where the nature of the secondary is unknown, are in agreement with the secondaries being wTTS rather than cTTS, and mixed pairs are very rare. Furthermore we analyse the X-ray emission of TTS systems resolvable by the ROSAT HRI. Among those systems we find statistical evidence that primaries show larger X-ray luminosity than secondaries, and that the samples of primary and secondary TTS are similar concerning the X-ray over bolometric luminosity ratios. Furthermore, primaries always emit harder X-rays than secondaries. In all cases where rotational velocities and/or periods are known for both companions, it is always the primary that rotates faster. Hence, the stronger X-ray emission of the primaries may be due to higher bolometric luminosity and/or faster rotation.
Astronomy and Astrophysics, v.369, p.971-980 (2001)Apr 2001
2000
- An X-ray flare from the Lindroos binary system HD 560
Abstract
We report on a large X-ray flare from the Lindroos binary system HD 560 during a pointed ROSAT PSPC observation. HD 560 is composed of a B9 and a G5 type star. The late-type companions in bound Lindroos binaries are Post T Tauri star (PTTS) candidates, i.e. pre-main sequence stars on their final approach to the main sequence. Strong (magnetic) activity is, therefore, expected for the G star, in contrast to the late-B type component for which no mechanism of X-ray production is known. The system is unresolved in the ROSAT image. During outburst the count rate has changed by at least a factor of 2, and presumably more (the maximum was not observed). The peak luminosity measured is ~ 1031 erg/s, comparable to the largest values derived for flares on pre-main sequence T Tauri stars. A two-temperature model for thermal emission from a hot, optically thin plasma shows an increase of the higher temperature as a result of coronal heating during the flare. The lower temperatures do not change during outburst.
Astronomy and Astrophysics, v.363, p.667-670 (2000)Nov 2000 - X-ray emission from young stars in the Tucanae association
Abstract
We report on X-ray emission from members of the recently discovered Tucanae association, a group of stars with youth signatures and similar space motion. The Tucanae association is the nearest known region of recent star formation ( ~ 45 pc) far from molecular clouds (\cite{Zuckerman00.1}). We have made use of the ROSAT Data Archive and searched for X-rays from Tucanae stars in both ROSAT All-Sky Survey (RASS) and pointed observations. While the RASS provides complete coverage of the sky, only three potential Tucanae members have been observed during PSPC pointings. All three stars have been detected. For the RASS the percentage of detections is 59%. The comparison of the X-ray luminosity function of Tucanae to that of other star forming regions may provide clues to the uncertain age of the association. We find that the distribution of X-ray luminosities is very similar to the ones derived for the TW Hya association, the Taurus-Auriga T Tauri Stars, and the IC 2602 cluster, but significantly brighter than the luminosity distribution of the Pleiades. We conclude that the stars in Tucanae are most likely young, on the order of 10-30 Myr. Strong variability of most stars emerges from the X-ray lightcurves where several flares and irregular variations are observed.
Astronomy and Astrophysics, v.361, p.581-593 (2000)Sep 2000 - RXJ 0529.4+0041: a low-mass pre-main sequence eclipsing-spectroscopic binary
Abstract
We report the discovery of a low-mass pre-main sequence eclipsing system among a sample of double-lined spectroscopic binaries found recently on the basis of ROSAT follow-up observations in the Orion star forming region. From the analysis of the available photometric and spectroscopic data, we derive orbital elements as well as fundamental stellar parameters for the individual components of the eclipsing PMS binary RXJ 0529.4+0041. These results provide a new observational test for theoretical PMS evolutionary models setting some useful constraints on currently available PMS tracks and isochrones. Based on observations carried out with the 2.2m telescope of Calar Alto Observatory (Spain), the Swiss Euler telescope (La Silla--Chile), the 0.9m telescope of Serra La Nave (Catania, Italy)
Astronomy and Astrophysics, v.361, p.L49-L52 (2000)Sep 2000 - X-ray emission from Lindroos binary systems
Abstract
We present a study of the X-ray emission from binary systems extracted from the Lindroos catalogue (Lindroos 1986) based on the ROSAT All-Sky survey as well as ROSAT PSPC and HRI pointings. The studied sample consists of visual binary systems comprised of early-type primaries and late-type secondaries. The ages of the systems were determined by Lindroos (1985) from uvbybeta photometry of the primaries. These ages range between 33 and 135 Myr, so if the late-type secondaries are physically bound to the early-type primaries, they could be Post-T Tauri stars (PTTS). We have found strong X-ray emission from several secondaries. This fact together with their optical and IR data, make them bona fide PTTS candidates. We have also detected X-ray emission from several early-type primaries and, in particular, from most of the late-B type stars. Because their HRI hardness ratios are similar to those from resolved late-type stars, the presence of an unresolved late-type companion seems to be the cause of this emission.
Astronomy and Astrophysics, v.359, p.227-241 (2000)Jul 2000 - A multi-wavelength study of pre-main sequence stars in the Taurus-Auriga star-forming region
Abstract
Although many lowmass pre-main sequence stars are strong X-ray sources, the origin of the X-ray emission is not well known. Since these objects are variable at all frequencies, simultaneous observations in X-rays and in other wavelengths are able to constrain the properties of the X-ray emitting regions. In this paper, we report quasi-simultaneous observations in X-rays, the optical, and the radio regime for classical and weak-line T Tauri stars from the Taurus-Auriga star-forming region. We find that all detected T Tauri stars show significant night-to-night variations of the X-ray emission. For three of the stars, FM Tau and CW Tau, both classical T Tauri stars, and V773 Tau, a weak-line T Tauri star, the variations are especially large. From observations taken simultaneously, we also find that there is some correspondence between the strength of Hα and the X-ray brightness in V773 Tau. The lack of a strong correlation leads us to conclude that the X-ray emission of V773 Tau is not a superposition of flares. However, we suggest that a weak correlation occurs because chromospherically active regions and regions of strong X-ray emission are generally related. V773 Tau was detected at 8.46 GHz as a weakly circularly polarised but highly variable source. We also find that the X-ray emission and the equivalent width of Hα remained unchanged, while large variations of the flux density in the radio regime were observed. This clearly indicates that the emitting regions are different. Using optical spectroscopy we detected a flare in Hα and event which showed a flare-like light-curve of the continuum brightness in FM Tau. However, ROSAT did not observe the field at the times of these flares. Nevertheless, an interesting X-ray event was observed in V773 Tau, during which the flux increased for about 8 hours and then decreased back to the same level in 5 hours. We interpret this as a long-duration event similar to those seen on the sun and other active stars. In the course of the observations, we discovered a new weak-line T Tauri star, GSC-1839-5674. Results are also presented for several other stars in the ROSAT field.
Astronomy and Astrophysics, v.357, p.206-218 (2000)May 2000 - X-ray flares on zero-age- and pre-main sequence stars in Taurus-Auriga-Perseus
Abstract
We present the results of a systematic search for X-ray flares on young stars observed during ROSAT PSPC observations of the Taurus-Auriga-Perseus sky region. All pointed PSPC observations currently available from the ROSAT Public Data Archive with known pre-main sequence T Tauri Stars or young Pleiads or Hyads in the field of view are analyzed. A study of the activity of late-type stars of different ages provides information on the evolution of their coronal activity, which may be linked to their angular momentum. We develop a criterion for the detection of flares based on the shape of the X-ray lightcurve. Applying our detection method to all 104 PSPC pointings from the archive we find 52 flares. Among them 15 are detected on T Tauri Stars, 20 on Pleiads, and 17 on Hyads. Only the 38 events which can definitely be attributed to late-type stars (i.e. stars of spectral type G and later) are considered in the statistical analysis of the properties of flaring stars. We investigate the influence of stellar parameters such as age, rotation and multiplicity on individual flare parameters and flare frequency. From the total exposure time falling to the share of each sample and the duration of the individual flares we compute a flare rate. We take into account that the detection sensitivity for large X-ray flares depends on the S/N and hence on the stellar distance. The values we derive for the flare rates are 0.86 +/- 0.16% for T Tauri Stars, 0.67 +/- 0.13% for Pleiads and 0.32 +/- 0.17% for Hyads. The flare rate of classical T Tauri Stars may be somewhat higher than that of weak-line T Tauri Stars (F_c = 1.09 +/- 0.39% versus F_w = 0.65 +/- 0.16%). Hardness ratios are used to track the heating that takes place during stellar flares. Hardness ratios are evaluated for three distinct phases of the flare: the rise, the decay, and the quiescent (pre- and post-flare) stage. In most cases the hardness increases during the flares as compared to the quiescent state. During both quiescence and flare phase TTSs display the largest hardness ratios, and the Hyades stars show the softest spectrum.
Astronomy and Astrophysics, v.356, p.949-971 (2000)Apr 2000 - The Mysterious X-Ray Emission from Late-B Type Stars: the Lindroos Sample
Abstract
We present a study of the X-ray emission coming from B-type stars in binary systems. The sample has been extracted from the Lindroos catalogue (Lindroos, 1986) and consists of six visual pairs comprised of a B-type star and a later-type companion. Surprisingly, we have detected X-ray emission coming from the late-B primaries, suggesting the presence of a third unresolved late-type star or an unknown mechanism which generates X-rays in late-B stars.
Astrophysics and Space Science, v. 272, Issue 1/3, p. 311-314 (2000).n/a 2000
1999
- Bayesian flare event detection. ROSAT X-ray observations of the UV Cetus type star G 131-026
Abstract
We have analyzed ROSAT PSPC observations for flare detection using a formalism of Bayesian model comparison. The method is applicable to the direct unbinned data and divides the whole observation into the most probable partitioning of non-uniform segments during which the photon arrival rate had no statistically significant variation. If more than one such segment is found with significantly different count rates, then variability is detected. As an example for this method, we show how a flare on the X-ray source RX J0008.8+2050, the counterpart of the UV Cet type star G 131-026, located in the solar vicinity, is detected and various items relevant to flare parameter estimation are discussed.
Astronomy and Astrophysics, v.345, p.121-126 (1999)May 1999 - Rotational modulation of X-ray flares on late-type stars: T Tauri stars and Algol
Abstract
We present evidence for rotational modulation of X-ray flares by an analysis of four outbursts on late-type stars. Two of the flares we discuss are found in ROSAT observations of T Tauri Stars and were obtained between September and October 1991. A flare on the T Tauri Star V773 Tau was observed by ASCA in September 1995. To this sample we add a Ginga observation of a flare on Algol observed in January 1989. The structure of the X-ray lightcurves observed in this selection of flare events is untypical in that the maximum emission extends over several hours producing a round hump in the lightcurve instead of a sharp peak. We explain this deviation from the standard shape of a flare lightcurve as the result of a flare erupting on the back side of the star and gradually moving into the line of sight due to the star
Astronomy and Astrophysics, v.344, p.154-162 (1999)Apr 1999 - Search for X-ray emission from bona-fide and candidate brown dwarfs
Abstract
Following the recent classification of the X-ray detected object V410 x-ray 3 with a young brown dwarf candidate (Briceño et al. 1998) and the identification of an X-ray source in Chamaeleon as young bona-fide brown dwarf (Neuhäuser & Comerón 1998), we investigate all ROSAT All-Sky Survey and archived ROSAT PSPC and HRI pointed observations with bona-fide or candidate brown dwarfs in the field of view with exposure times ranging from 0.13 to 221 ks, including dedicated 64 ks and 42 ks deep ROSAT HRI pointed observations on the low-mass star BRI 0021-0214 and the brown dwarf Calar 3, respectively. Out of 26 bona-fide brown dwarfs, one is newly detected in X-rays, namely rho Oph GY 202. Also, four out of 57 brown dwarf candidates studied here are detected in X-rays, namely the young Taurus brown dwarf candidates MHO-4, MHO-5, V410 Anon 13, and V410 x-ray 3. The M9.5-type star BRI 0021-0214 is not detected. In the appendix, we also present catalogued, but as yet unnoticed B- and R-band data for some of the objects studied here.
Astronomy and Astrophysics, v.343, p.883-893 (1999)Mar 1999 - Evolution of spectral parameters during a pre-eclipse dip of HER X-1
Abstract
We report on a pre-eclipse dip of the X-ray binary pulsar Her X-1 observed by the Rossi X-ray Timing Explorer (RXTE) in 1996 July. The energy spectra in the 3-18 keV range can be described by the sum of two power-laws, one of which is modified by photoelectric absorption and by Thomson scattering in cold material, plus an iron emission line at 6.7 keV. We present the evolution of the spectral parameters with a temporal resolution of 16 s and show that the varying flux and spectrum can be interpreted solely by a time varying column density. The data do not appear to require non-solar abundances in the absorbing material, although a slight over-abundance of the metals cannot be ruled out. We also find that the lightcurve is characterized by symmetric substructures which can be successfully modeled by Gaussian profiles. The recurrence time of these substructures is on a timescale of a few minutes. Table~2 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/Abstract.html.
Astronomy and Astrophysics, v.342, p.736-744 (1999)Feb 1999
Books
2008
- Chapter 10. on Nuclear burning starsIn: The Universe in X-rays, eds. J.E. Trümper & G. Hasinger, 2008, Springer, 498 pages 237 illustrations, 40 in color, Hardcover - ISBN: 978-3-540-34411-72008
Not Refereed
2016
- A Path Towards Understanding The Rotation-Activity-Age Relation Of M Dwarfs With K2 Mission, X-Ray And Uv Data
Abstract
We present an updated view of the rotation-activity-age relation for (old) field M dwarfs based on K2 rotation periods and flares,archival X-ray and UV data,and dedicated Chandra and XMM-Newton observations.The rotation-activity-age relation can be used as a proxy for magnetic fields- which are difficult to measure in M dwarfs -and is, therefore, key to studies of(i) the predicted dynamo transition at the fully convective boundary(SpT M3),(ii) differences in angular momentum loss through magnetized windswith respect to solar-type stars,and (iii) the evaporation of planet atmospheres,especially relevant for M dwarfs because ofthe small separation of their planets
The 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (CS19), Uppsala, Sweden, 06-10 June 2016, id.62Jul 2016 - Cool star X-ray variability
Abstract
Variability is a key characteristic of late-type stars. In analogy to the Sun, late-type stars display a range of magnetic activity phenomena. These comprise strong radiation in the X-ray band emerging from the stellar corona as a result of magnetic heating. The time-scales of the observed X-ray variability associated with magnetic activity range from hours (for flares) to years (for dynamo cycles). Next to these activity-related variability features, in Young Stellar Objects (YSO) the mass accretion from a circumstellar disk and protostellar outflows can induce X-ray emission. The YSO circumstellar environment can give rise to variability either due to intrinsic changes in mass transfer or due to geometric effects as accretion streams or structures in the disk rotate in and out of the line-of-sight. Magnetic interaction between star and disk may play a role as well. I summarize recent developments in this research area and point out some directions for the possible contributions of XMM-Newton in the future.
XMM-Newton: The Next Decade, Proceedings of the Conference held 9-11 May, 2016 at ESAC, Madrid. Online at http://www.cosmos.esa.int/web/xmm-newton/2016-workshop, id.9Jun 2016 - Magnetic Activity of Ultracool Dwarfs
Abstract
Ultracool dwarfs are very low mass objects that have a temperature below about 2,800K; magnetic activity signatures probe magnetic field behavior in a mass regime intermediate between stars and planets. Very few ultracool dwarfs have detected levels of radio emission, so increasing the sample size is important for the understanding the variety of magnetic processes at work. Because of the dramatic variations in radio and X-ray properties, these probes are best done with simultaneous measurements. We report on a small sample of ultracool dwarfs that have been observed simultaneously with XMM Newton Observatory and the Jansky Very Large Array, for constraints on their coronal plasmas through X-ray and radio observations, respectively; we present the results of the radio reduction, calibration and imaging of our sample. We discuss the implications of upper limits and detections for increasing our understanding of magnetic field behavior in this regime.
American Astronomical Society, AAS Meeting #227, id.145.15Jan 2016 - Young Stars & Planets Near the Sun,
Abstract
Not Available
Young Stars & Planets Near the Sun, Proceedings of the International Astronomical Union, IAU Symposium, Volume 314, 2016.Jan 2016 - Accretion, Disks, and Magnetic Activity in the TW Hya Association
Abstract
We present new photometric and spectroscopic data for the M-type members of the TW Hya association with the aim of a comprehensive study of accretion, disks and magnetic activity at the critical age of ~ 10 Myr where circumstellar matter disappears.
Young Stars & Planets Near the Sun, Proceedings of the International Astronomical Union, IAU Symposium, Volume 314, pp. 105-108Jan 2016
2014
- X-ray emission of the young stellar population of the Orion B molecular cloud
Abstract
We carried out an analysis of nine XMM-Newton archive observations covering a significant part of the Orion B molecular Cloud. We completed the analysis by using Infrared (Spitzer, WISE, and 2MASS) and Optical (Optical Monitor and UCAC4) photometry data. This work is focused on the classification and characterization of young stellar objects and the inhomogeneity along the cloud. From nine X-ray observations we detected 604 sources in which 490 of them have at least a counterpart. We obtained the X-ray coronal properties of 49 sources with more than 100 net counts, using the 1T or 2T-model of XSPEC. After rejecting the background sources we classified the sample in 332 Classes III, 141 Classes II and 11 Classes 0/I based on their infrared properties. We explored the differences along the cloud and discovered 5 different groups where Class 0/I and Class II objects are located, coincident with NGC2023, NGC2024, NGC2068, NGC2071 and around V1647-Ori. We compared the X-ray luminosity functions for the different classes obtained, and also the complete sample with the COUP distribution, which reveals similar populations.
The X-ray Universe 2014, edited by Jan-Uwe Ness. Online at http://www.cosmos.esa.int/web/xmm-newton/2014-symposium/, id.277Jul 2014 - The stellar population in the XMM-Newton slew survey
Abstract
We study X-ray transients from the XMM-Newton slew catalog with focus on the identification of flare stars. We have selected 118 variable X-ray sources from the XMM-Newton slew catalog Delta-3 on the basis of at least 70-fold enhanced count rate with respect to ROSAT. Our characterization of the counterparts includes a systematic cross-match with optical and infrared photometric catalogs. The photometry was used to identify candidate stars for spectroscopic follow-up. Optical low-resolution spectra were obtained for 34 of those objects with OSIRIS@GranTeCan, most of them without any previous spectroscopic data. Among these objects we find many M-type stars, some giant stars and a small number of extragalactic sources. We present here the X-ray, optical and near-infrared properties of all slew transients from our sample together with the statistics of the various source populations.
The X-ray Universe 2014, edited by Jan-Uwe Ness. Online at http://www.cosmos.esa.int/web/xmm-newton/2014-symposium/, id.321Jul 2014 - Magnetic fields of Herbig Ae/Be stars
Abstract
We report on the status of our spectropolarimetric studies of Herbig Ae/Be stars carried out during the last years. The magnetic field geometries of these stars, investigated with spectropolarimetric time series, can likely be described by centred dipoles with polar magnetic field strengths of several hundred Gauss. A number of Herbig Ae/Be stars with detected magnetic fields have recently been observed with X-shooter in the visible and the near-IR, as well as with the high-resolution near-IR spectrograph CRIRES. These observations are of great importance to understand the relation between the magnetic field topology and the physics of the accretion flow and the accretion disk gas emission.
Physics at the Magnetospheric Boundary, Geneva, Switzerland, Edited by E. Bozzo; P. Kretschmar; M. Audard; M. Falanga; C. Ferrigno; EPJ Web of Conferences, Volume 64, id.08006Jan 2014 - Emission line diagnostics for accretion and outflows in young very low-mass stars and brown dwarfs
Abstract
We discuss accretion and outflow properties of three very low-mass young stellar objects based on broad-band mid-resolution X-Shooter/VLT spectra. Our targets (FU Tau A, 2M1207-39, and Par-Lup3-4) have spectral types between M5 and M8, ages between 1Myr and ~ 10Myr, and are known to be accreting from previous studies. The final objective of our project is the determination of mass outflow to accretion rate for objects near or within the substellar regime as a probe for the T Tauri phase of brown dwarfs and the investigation of variability in the accretion and outflow processes.
Physics at the Magnetospheric Boundary, Geneva, Switzerland, Edited by E. Bozzo; P. Kretschmar; M. Audard; M. Falanga; C. Ferrigno; EPJ Web of Conferences, Volume 64, id.08005Jan 2014 - An X-Shooter Analysis of Chromospheric Activity of Class III Low Mass Sources
Abstract
The knowledge of the photospheric parameters and the level of chromospheric activity in young pre-main sequence stars is one of the main limitations when trying to measure mass accretion rates in Class II YSOs. A detailed characterization of photospheres and chromospheric activities in low-mass, young stars without disks (late K and M type Class III YSOs) is still missing. Using VLT/X-Shooter spectra we have analyzed a sample of Class III in the Spectral Type range between K5 and M9.5. We report on the characterization of the chromospheric emission in the stars in our sample and on the implications that our work has on accurate measurements of mass accretion rates in YSOs.
The Labyrinth of Star Formation, Astrophysics and Space Science Proceedings, Volume 36. ISBN 978-3-319-03040-1. Springer International Publishing Switzerland, 2014, p. 45n/a 2014
2013
- X-Shooter spectroscopy of brown dwarfs in the TW Hya association
Abstract
We present broad-band mid-resolution X-Shooter/VLT spectra for four brown dwarfs of the TW Hya association. Our targets comprise substellar analogs representing the different evolutionary phases in young stellar evolution. In the two diskless brown dwarfs we determine the stellar parameters and we study their chromospheric emission line spectrum. For the two accreting brown dwarfs we estimate the mass accretion rates.
Memorie della Societa Astronomica Italiana, v.84, No. 4, p.1107 (2013)n/a 2013
2012
- Magnetic field studies of Herbig Ae/Be stars
Abstract
We report on the status of our spectropolarimetric studies of Herbig Ae/Be stars carried out during the last years. For the first time we examined the relation of the measured field strengths with various parameters that characterise the star-disk system. We could show that strongermagnetic fields tend to be found in younger Herbig stars and completely disappear when stars arrive on the ZAMS, indicating that the magnetic fields of Ap/Bp stars are not fossil. New determinations of the mean longitudinal magnetic field for several Herbig Ae/Be stars are presented. The longitudinal magnetic field measurements of MWC480 reveal the presence of a strong kG field, which was undetected in our previous low-resolution polarimetric observations with FORS 1. The magnetic field geometries of Herbig Ae/Be stars studied with spectropolarimetric time series can likely be described by centred dipoles with polar magnetic field strengths of several hundred Gauss. The sharp-lined young Herbig Ae star HD101412 with a strong surface magnetic field has become in the last years one of the best-studied targets among the Herbig Ae/Be stars.
STELLAR POLARIMETRY: FROM BIRTH TO DEATH. AIP Conference Proceedings, Volume 1429, pp. 21-28 (2012).May 2012
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 - First results from XILO: XMM-Newton Investigations in the Lambda Orionis star forming region
Abstract
The λ Orionis star formation region (1-6 Myr, 400 pc) is a complex of star-forming clouds surrounded by a molecular ring with ~ 5° radius which was probably formed by a supernova explosion (Dolan & Mathieu 2002). For a complete picture of star formation, believed to be determined by the supernova blast, the large-scale distribution of the pre-main sequence population in λ Ori needs to be examined. We have embarked on a multi-wavelength study (XMM-Newton/X-ray, CFHT/optical, Spitzer/IR) of selected areas within this intriguing star-forming complex that enables us to identify young stars and brown dwarfs. Our study comprises various areas within the cloud complex as shown in Fig.1. This data set is among the most extended X-ray surveys carried out with XMM-Newton in a coherent star-forming environment. The XMM-Newton observations combined with optical and IR data reveal the low-mass stellar population down to ~ 0.4 M⊙. For this mass-limited sample, our preliminary analysis confirms the anomalously low disk-fraction of the central star cluster Coll 69, the Eastern extension of its low-mass population pointing towards B 35, and the concentration of young stars in front of B 35. The analysis of the ‘on-cloud field
Highlights of Astronomy, Volume 15, p. 768-768Nov 2010
2009
- A Multiwavelength Approach to the Star Formation in the Orion
Abstract
The O8 III star Lambda Orionis is the dominant object in the Orion
The Eighth Pacific Rim Conference on Stellar Astrophysics: A Tribute to Kam-Ching Leung ASP Conference Series, Vol. 404, proceedings of the conference held 5-9 May, 2008, at Merlin Beach Hotel, Phuket, Thailand. Edited by B. Soonthornthum, S. Komonjinda, K.S. Cheng, and K.C. Leung San Francisco: Astronomical Society of the Pacific, 2009., p.9Aug 2009 - Studying the Magnetic Properties of Upper Main-sequence Stars with FORS1
Abstract
We summarise the results of our recent magnetic field studies in upper main-sequence stars, which have exploited the spectropolarimetric capability of FORS1 at the VLT extensively.
The Messenger, vol. 135, p. 21-25Mar 2009 - Coronal activity with XMM-Newton and Chandra
Abstract
XMM-Newton and Chandra have greatly deepened our knowledge of stellar coronae giving access to a variety of new diagnostics such that nowadays a review of stellar X-ray astronomy necessarily must focus on a few selected topics. Attempting to provide a limited but representative overview of recent discoveries I discuss three subjects: the solar-stellar connection, the nature of coronae in limiting regimes of stellar dynamos, and ``hot topics
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. 190-199 (2009).Feb 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 - Mass-Loss and Magnetic Fields as Revealed Through Stellar X-ray Spectroscopy
Abstract
Not Available
Astro2010: The Astronomy and Astrophysics Decadal Survey, Science White Papers, no. 228n/a 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 - Highlights from the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)
Abstract
The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) is a large program designed to systematically investigate the X-ray properties of young stellar/substellar objects in the Taurus star forming region. This paper summarizes selected key findings from XEST.
14th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun ASP Conference Series, Vol. 384, proceedings of the conference held 5-10 November, 2006, at the Spitzer Science Center and Michelson Science Center, Pasadena, California, USA. Edited by Gerard van Belle., p.65Apr 2008 - Search for Low-mass Companions to X-ray Emitting A-type Stars
Abstract
There is no obvious theory that would explain X-ray emission from main-sequence A-type stars. Therefore, the X-ray emission identified with these stars on basis of low spatial resolution X-ray observations is usually attributed to magnetic activity from unknown late-type companion stars. We systematically study the literature and public 2MASS data in search for binaries among A stars. This way, we identify new candidate counterparts for the X-ray sources in at least 60% of the A-type stars claimed to be detected in the ROSAT All-Sky Survey. There is marginal evidence that this fraction decreases from early to late A spectral type, possibly indicating the onset of intrinsic X-ray emission for the latest A stars which may have deep enough convection zones to support magnetic activity.
Multiple Stars Across the H-R Diagram, ESO Astrophysics Symposia. ISBN 978-3-540-74744-4. Springer-Verlag Berlin Heidelberg, 2008, p. 299n/a 2008
2007
- Accretion and outflow-related X-rays in T Tauri stars
Abstract
We report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the “XMM-Newton Extended Survey of the Taurus Molecular Cloud.” X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a “soft excess” in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion streams.
Star-Disk Interaction in Young Stars, Proceedings of the International Astronomical Union, IAU Symposium, Volume 243, p. 155-162May 2007
2006
- Age, mass and temperature dependence of X-ray activity on brown dwarfs .
Abstract
We present Chandra observations of three brown dwarfs in different evolutionary stages. Combining our data with X-ray data on brown dwarfs from the literature yields a sample that covers the age range from ˜ 1 Myr to ˜ 1 Gyr. Since brown dwarfs cool and dim as they age, this allows us to study the influence of various stellar parameters (T_eff, L_bol, mass, age) onto their X-ray activity.
Memorie della Società Astronomica Italiana Supplement, v.9, p.273 (2006)n/a 2006 - A Survey of Young Stars in the Taurus Molecular Cloud with XMM-Newton
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.39Jan 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
- VizieR Online Data Catalog: XMM-Newton obs, in Cha I South (Stelzer+, 2004)
Abstract
A 30ks XMM-Newton observation centered on the candidate brown dwarf ChaHα 3 was carried out on April 9, 2002 with the European Photon Imaging Camera (EPIC) as prime instrument. (4 data files).
VizieR On-line Data Catalog: J/A+A/423/1029. Originally published in: 2004A&A...423.1029SSep 2005 - Studies of the coronae of the lowest-mass stars and brown dwarfs in the XMM-Newton and Chandra era
Abstract
Not Available
EPIC-XMM-Newton Consortium Meeting - 5 years of Science with XMM-Newton : Proceedings of the 2005 EPIC XMM-Newton Consortium Meeting, Schloss Ringberg, April 11-13 2005; Max-Planck-Insitut für extraterrestrische Physik, MPE Report 288, p.17-20Jun 2005 - Multiwavelength study of X-ray emitting A- and B-stars testing the companion hypothesis
Abstract
We report on the first results of a multi-wavelength approach to test the hypothesis that the X-ray emission from intermediate-mass stars is generated by late-type magnetically active companions. Our high spatial resolution observations of A/B-type stars in the IR using adaptive optics uncover hypothetical companion stars at arcsecond separations from the primary. The same targets are then followed-up in X-rays with Chandra to pinpoint the X-ray source. Finally, those A/B-type stars that are still detected with Chandra are studied with IR spectroscopy, where we search for temperature sensitive features indicating the existence of even closer cool companions.
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.213Mar 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
2003
- A Search for Disk Emission in Young Brown Dwarfs: L
Abstract
Studies of disks around young brown dwarfs are of paramount importance to our understanding of the origin, diversity and early evolution of sub-stellar objects. Here we present first results from a systematic search for disk emission in a spectroscopically confirmed sample of young objects near or below the sub-stellar boundary in a variety of star-forming regions. Our VLT and Keck L
Brown Dwarfs, Proceedings of IAU Symposium #211, held 20-24 May 2002 at University of Hawaii, Honolulu, Hawaii. Edited by Eduardo Martín. San Francisco: Astronomical Society of the Pacific, 2003, p. 139Jun 2003 - X-ray Emission from Old and Intermediate Age Brown Dwarfs
Abstract
We report on two recent XMM-Newton observations of Brown Dwarfs in the Pleiades cluster and in the field aiming to constrain the age dependence of X-ray emission from substellar objects.
Brown Dwarfs, Proceedings of IAU Symposium #211, held 20-24 May 2002 at University of Hawaii, Honolulu, Hawaii. Edited by Eduardo Martín. San Francisco: Astronomical Society of the Pacific, 2003, p. 443Jun 2003 - X-ray emission from NGC 2362
Abstract
Not Available
Open Issues in Local Star Formation. Proceedings of the Ouro Preto Colloquium, Brazil, April 5-10, 2003. Edited by Jacques Lépine and Jane Gregorio-Hetem. Astrophysics and Space Science Library, Volume 299. Kluwer Academic Publishers, Dordrecht, 2003., p.199n/a 2003 - Accretion signatures in the X-ray spectrum of TW Hya
Abstract
Not Available
Open Issues in Local Star Formation. Proceedings of the Ouro Preto Colloquium, Brazil, April 5-10, 2003. Edited by Jacques Lépine and Jane Gregorio-Hetem. Astrophysics and Space Science Library, Volume 299. Kluwer Academic Publishers, Dordrecht, 2003., p.177n/a 2003
2002
- The joint XMM-Newton and Chandra view of YY Gem
Abstract
We have observed the flare star YY Gem simultaneously with XMM-Newton and Chandra as part of a multi-wavelength campaign aiming at a study of variability related to magnetic activity in this short-period eclipsing binary. Here we report on the first results from the analysis of the X-ray spectrum. The vicinity of the star provides high enough S/N in the CCD cameras onboard XMM-Newton to allow for time-resolved spectroscopy. Since the data are acquired simultaneously they allow for a cross-calibration check of the performance of the XMM-Newton RGS and the LETGS on Chandra.
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.215Dec 2002 - The High Resolution X-ray Spectrum of Castor A and Castor B from a Simultaneous XMM-Newton and Chandra Observation
Abstract
We present a simultaneous Chandra and XMM-Newton observation of the Castor sextett, focusing on Castor A and Castor B. Our study represents the first unambiguous X-ray detection of all three visual components in the Castor sextett making use of the unprecedented spatial resolution of Chandra. Of the present day X-ray instruments only Chandra can isolate the X-ray lightcurves and spectra of Castor A and B (angular separation ~4"). We compare the Chandra observation with XMM-Newton data obtained simultaneously. Albeit not able to resolve Castor A and Castor B from each other, the higher sensitivity of XMM-Newton allows for a quantitative analysis of their combined high-resolution spectrum. We use He-like line triplets to examine the temperature and the density in the corona of Castor AB, and find n_e = (0.5...1) x 10^10 cm^-3 (for oxygen) and n_e = 2 x 10^12 cm^-3 (for neon). The latter one is likely to be too high due to contamination of the Ne IX triplet with iron lines. Our comparison with the conditions of the coronal plasma of other stars shows that Castor AB behave like typical late-type coronal X-ray emitters supporting the common notion that the late-type secondaries within each spectroscopic binary are the sites of X-ray production.
High Resolution X-ray Spectroscopy with XMM-Newton and Chandra, Proceedings of the international workshop held at the Mullard Space Science Laboratory of University College London, Holmbury St Mary, Dorking, Surrey, UK, October 24 - 25, 2002, Ed. Branduardi-Raymont, G., published electronically and stored on CD., meeting abstractDec 2002 - The XMM-Newton View of TW Hya
Abstract
We present the X-ray spectrum of TW Hya observed at high spectral resolution with the Reflection Grating Spectrometer onboard the XMM-Newton satellite. We determine line fluxes for individual emission lines in the high resolution spectrum, and find that the predominant emission is from neon and oxygen, with comparatively weak iron lines. No emission from magnesium and silicon is detected. Comparison to a Chandra HETGS observation carried out about one year before shows that TW Hya exhibits moderate, irregular variability. Unlike all other high-resolution X-ray spectra of late-type stars obtained so far the flux ratio between the forbidden f and the intercombination i line of helium-like ions is very low (R = f/i 10^12 cm-3, while the density in typical stellar coronae is about two orders of magnitude lower. Together with the lack of Mg and Si emission this provides strong evidence that its X-ray emission is produced in a disk-fed accretion column (depleted of grain-forming elements) rather than by magnetic activity.
High Resolution X-ray Spectroscopy with XMM-Newton and Chandra, Proceedings of the international workshop held at the Mullard Space Science Laboratory of University College London, Holmbury St Mary, Dorking, Surrey, UK, October 24 - 25, 2002, Ed. Branduardi-Raymont, G., published electronically and stored on CD., meeting abstractDec 2002 - Coordinated Multiwavelength Observations of V410Tau
Abstract
In November 2001 we undertook a coordinated observing campaign to study the connection between X-ray and optical variability in the weak-line T Tauri star V410Tau. The observing plan included three 15 ksec observations with Chandra using the Advanced CCD Imaging Spectrometer for Spectroscopy scheduled for different phases of the known 1.87 d starspot cycle. Photometric and spectroscopic monitoring of V410Tau involving telescopes on three different continents was scheduled simultaneously with the Chandra exposures.
eprint arXiv:astro-ph/0206110Jun 2002
2001
- New insights on the accretion disk corona of Her X-1 from Chandra LETGS data
Abstract
The X-ray binary Her X-1 was observed using the Chandra low energy transmission grating spectrometer (LETGS) during its anomalous low state and 10 months later during the X-ray bright phase. These observations allow us for the first time to detect emission, in the form of the intercombination lines of density sensitive He-like OVII and NVI triplets as well as the Lya-like OVIII and NVII lines, from the corona above the accretion disc in Her X-1. The He-like emission lines seem to be present at all times. Both data sets cover nearly the same range in orbital phase which allows for good comparison of properties in the different states. Additionally the second data set shows clearly a series of pre-eclipse dips. The spin period was not detected in the anomalous low state but clearly in the X-ray 35-day period bright data where the period we obtain indicates that the spin-period is continuing to increase. A detailed analysis of these fascinating observations of Her X-1 will be presented here.
Two Years of Science with Chandra, Abstracts from the Symposium held in Washington, DC, 5-7 September, 2001, meeting abstract.Sep 2001 - X-ray Flares on Young Late-Type Stars in Taurus-Auriga-Perseus (CD-ROM Directory: contribs/stelzer)
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.1171n/a 2001 - X-ray Emission from Lindroos Binary Systems: Searching for Post-T Tauri Stars (CD-ROM Directory: contribs/huelamo)
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.1438n/a 2001 - X-ray Luminosity Functions of Young Stars in Taurus-Auriga-Perseus
Abstract
X-ray luminosity functions (XLF) are frequently used to assess the properties of the X-ray emission from a given sample of stars, and particularly important for a comparison of different groups of stars. I present the results from a detailed analysis of the XLF of pre-main sequence T Tauri Stars in Taurus-Auriga and young main-sequence objects (represented by the Pleiades and Hyades cluster). The study is based on the ROSAT public archive. Various possible biases related to the sample selection are described, and their influence on the luminosity distributions is discussed.
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.165n/a 2001 - X-ray Emission and Variability of Young, NearbyStars
Abstract
X-ray emission has played a major role in the recent identification of several groups of young, nearby stars. The ROSAT All-Sky Survey (RASS) is due to its spatial completeness an important tool for discovering stellar associations and investigating their X-ray properties. However, it has comparatively low sensitivity and poor temporal coverage, and is therefore of limited use for variability studies. The situation is greatly improved by the new generation of X-ray observatories, e.g. for coronal X-ray sources XMM has about an order of magnitude higher sensitivity than the RASS and provides much longer exposure times allowing continuous monitoring for more than 40 h. We demonstrate the capabilities of both the RASS by comparing X-ray luminosity functions (XLF) of different young stellar associations, and XMM by discussing one young very nearby star in detail, GJ 182.
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.153n/a 2001 - X-ray Emission of Multiple T Tauri Stars in Taurus
Abstract
Not Available
Magnetic Fields Across the Hertzsprung-Russell Diagram, ASP Conference Proceedings Vol. 248. Edited by G. Mathys, S. K. Solanki, and D. T. Wickramasinghe. ISBN: 1-58381-088-9. San Francisco: Astronomical Society of the Pacific, 2001., p.531n/a 2001 - The PMS Eclipsing Binary RXJ 0529.4+0041
Abstract
We report the discovery of the first low--mass pre--main sequence eclipsing binary among a sample of double-lined spectroscopic binaries in the Orion star forming region found in a previous high-resolution spectroscopic investigation on ROSAT--discovered weak-T Tauri stars. Here we present the preliminary results from the combined analysis of the spectroscopic orbit and B and V light--curves, using data available till spring 2000. We then compare the fundamental stellar parameters derived from the orbital solution with those inferred from some widely used theoretical evolutionary models.
The Formation of Binary Stars, Proceedings of IAU Symp. 200, held 10-15 April 2000, in Potsdam, Germany, Edited by Hans Zinnecker and Robert D. Mathieu, 2001, p. 468.n/a 2001
2000
- Search for low-mass pre-main sequence eclipsing binaries.
Abstract
We announce the discovery of a low--mass pre--main sequence eclipsing binary among a sample of double-lined spectroscopic binaries found in the Orion star forming region in a previous high-resolution spectroscopic investigation on ROSAT--discovered weak-T Tauri stars (Alcalá et al. 2000). As our observing programme is presently going-on, we report here some preliminary results based on data collected till January 2000.
Birth and Evolution of Binary Stars, Poster Proceedings of IAU Symposium No. 200 on The Formation of Binary Stars, held 10-15 April, 2000, in Potsdam, Germany. Edited by Bo Reipurth and Hans Zinnecker, 2000, p. 9.n/a 2000 - Multiwavelength observations of Lindroos binary systems
Abstract
We present a study of the X-ray emission from Lindroos binary systems as observed by the ROSAT satellite. These binary systems are visual pairs comprised of an early-type primary and a late-type secondary. Their ages range between 10 and 100 Myrs so, if physically bound, the secondary stars are Post-T Tauri star candidates. We have detected strong X-ray emission from most of the Lindroos secondaries under study. Their X-ray emission is comparable to that reported for young late-type stars. This fact together with the presence of the Li I (6708 Å ) absorption line in their spectra, provides clear evidence of youth among the Lindroos sample. New optical and infrared data of these objects have been already taken and will allow us to estimate more precisely their ages and their spectral energy distributions.
Birth and Evolution of Binary Stars, Poster Proceedings of IAU Symposium No. 200 on The Formation of Binary Stars, held 10-15 April, 2000, in Potsdam, Germany. Edited by Bo Reipurth and Hans Zinnecker, 2000, p. 88.n/a 2000
1999
- X-ray Flares in Taurus-Auriga-Perseus
Abstract
Not Available
Optical and Infrared Spectroscopy of Circumstellar Matter, ASP Conference Series, Vol. 188. Edited by Eike Guenther, Bringfried Stecklum, and Sylvio Klose. ISBN: 1-58381-014-5 (1999), p.151n/a 1999 - X-ray Flares on the UV Cet Type Stars
Abstract
Not Available
Optical and Infrared Spectroscopy of Circumstellar Matter, ASP Conference Series, Vol. 188. Edited by Eike Guenther, Bringfried Stecklum, and Sylvio Klose. ISBN: 1-58381-014-5 (1999), p.155n/a 1999 - Her X-1: Spectral parameters during a pre-eclipse dip
Abstract
Not Available
Highlights in x-ray astronomy : international symposium in honour of Joachim Trümpern/a 1999