B. Stelzer

The Origin of T Tauri X-Ray Emission: New Insights from the Chandra Orion Ultradeep Project

The Chandra Orion Ultradeep Project (COUP) provides the most comprehensive data set ever acquired on the X-rayemissionofpre–main-sequencestars.Inthispaper,westudythenearly600X-raysourcesthatcanbereliably identified with optically well-characterized T Tauri stars (TTSs) in the Orion Nebula Cluster. With a detection limit of LX; min � 10 27:3 ergs s � 1 for lightly absorbed sources, we detect X-ray emission from more than 97% of the optically visible late-type (spectral types F–M) cluster stars. This proves that there is no ‘‘X-ray–quiet’’ population of late-type stars with suppressed magnetic activity. We use this exceptional optical, infrared, and X-ray data set tostudythe dependenciesoftheX-rayproperties onotherstellarparameters. AllTTSs withknownrotationperiods lie in the saturated or supersaturated regime of the relation between activity and Rossby numbers seen for mainsequence (MS) stars, but the TTSs show a much larger scatter in X-ray activity than that seen for the MS stars. Strong near-linear relations between X-ray luminosities, bolometric luminosities, and mass are present. We also

The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)

(abridged:) The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) surveys the most populated ~5 square degrees of the Taurus star formation region, using the XMM-Newton X-ray observatory to study the thermal structure, variability, and long-term evolution of hot plasma, to investigate the magnetic dynamo, and to search for new potential members of the association. Many targets are also studied in the optical, and high-resolution X-ray grating spectroscopy has been obtained for selected bright sources. The X-ray spectra have been coherently analyzed with two different thermal models (2-component thermal model, and a continuous emission measure distribution model). We present overall correlations with fundamental stellar parameters that were derived from the previous literature. A few detections from Chandra observations have been added. The present overview paper introduces the project and provides the basic results from the X-ray analysis of all sources detected in the XEST survey.Comprehensive tables summarize the stellar properties of all targets surveyed. The survey goes deeper than previous X-ray surveys of Taurus by about an order of magnitude and for the first time systematically accesses very faint and strongly absorbed TMC objects. We find a detection rate of 85% and 98% for classical and weak-line T Tau stars (CTTS resp. WTTS), and identify about half of the surveyed protostars and brown dwarfs. Overall, 136 out of 169 surveyed stellar systems are detected. We describe an X-ray luminosity vs. mass correlation, discuss the distribution of X-ray-to-bolometric luminosity ratios, and show evidence for lower X-ray luminosities in CTTS compared to WTTS. Detailed analysis (e.g., variability, rotation-activity relations, influence of accretion on X-rays) will be discussed in a series of accompanying papers.

X-shooter spectroscopy of young stellar objects - IV. Accretion in low-mass stars and substellar objects in Lupus

We present X-Shooter/VLT observations of a sample of 36 accreting low-mass stellar and sub-stellar objects (YSOs) in the Lupus star forming region, spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun < Mstar < 0.5Msun. Our aim is twofold: firstly, 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 from it the accretion rate (Macc), is derived by modelling the excess emission, from the UV to the near-IR, as the continuum emission of a slab of hydrogen. The flux and luminosity (Ll) of a large number of emission lines of H, He, CaII, etc., observed simultaneously in the range from ~330nm to 2500nm, were computed. 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 as compared to previous relationships in the literature. Our measurements extend the Pab and Brg relationships to Lacc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies to measure Lacc and Macc yield significantly different results: Ha line profile modelling may underestimate Macc by 0.6 to 0.8dex with respect to Macc derived from continuum-excess measures. Such differences may explain the likely spurious bi-modal relationships between Macc and other YSOs properties reported in the literature. We derive Macc in the range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to Mstar^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 Macc.

X-ray emission from a metal depleted accretion shock onto the classical T Tauri star TW Hya

We present the X-ray spectrum of TW Hya observed at high and in termediate 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-r ay data with both high spectral resolution and high sensitiv ity were obtained, thus allowing to probe the X-ray emission propert ies of stars in the early pre-main sequence phase. Despite TW Hya’s high X-ray luminosity in excess of 1030 erg/s its X-ray spectrum is dominated by emission lines from ather cool plasma (T 3 MK), and only little emission measure is present at high tem p ratures (T 10 MK). We determine photon fluxes for the emission lines in the high resolution spectrum, confirming t he earlier result fromChandra that the predominant emission is from neon and oxygen, with comparatively weak iron lines. Fu rther, the line ratios of He-like triplets of nitrogen, oxyg en and neon require densities of ne 1013 m 3, about two orders of magnitude higher than for any other star observed so far at high spectral resolution. Finally, we find that nearly all me tals are underabundant with respect to solar abundances, wh ile the abundances of nitrogen and neon are enhanced. The high plasm a density, the (comparatively) low temperature, and peculi ar chemical abundances in the X-ray emitting region on TW Hya ar e untypical for stellar coronae. An alternative X-ray produ ction mechanism is therefore called for and a natural explanation is an accretion column depleted of grain forming elements. T he metal depletion could be either due to the original molecula r c oud that formed TW Hya or due to a settling of dust in the circumstellar disk of TW Hya.

X-RAY EMISSION FROM EARLY-TYPE STARS IN THE ORION NEBULA CLUSTER

The X-ray properties of twenty ~1 Myr old O, B, and A stars of the Orion Trapezium are examined with data from the Chandra Orion Ultradeep Project (COUP). On the basis of simple theories for X-ray emission, we define two classes separated at spectral type B4: hotter stars have strong winds that may give rise to X-ray emission in small- or large-scale wind shocks, and cooler stars that should be X-ray dark due to their weaker winds and absence of outer convection zones where dynamos can generate magnetic fields. Emission by late-type magnetically active companions may be present in either class. Sixteen of the 20 stars are detected with a wide range of X-ray luminosities, log LX ~ 29-33, and X-ray efficiencies, log(LX/Lbol) ~ -4 to -8. Only two stars, ?1 Ori D (B0.5) and NU Ori (B1), show exclusively the constant soft-spectrum emission at log(LX/Lbol) ~ -7 expected from the standard model involving many small shocks in an unmagnetized radiatively accelerated wind. Most of the other massive O7-B3 stars exhibit some combination of soft-spectrum wind emission, hard-spectrum flaring, and/or rotational modulation indicating large-scale inhomogeneity. Magnetic confinement of winds with large-scale shocks can be invoked to explain these phenomena. This is supported in some cases by nonthermal radio emission and/or chemical peculiarities, or direct detection of the magnetic field (?1 Ori C). Most of the stars in the weak-wind class exhibit X-ray flares and log LX < 31 ergs s-1, consistent with magnetic activity from known or unseen low-mass companions. In most cases, the X-ray spectra can be interpreted in terms of a two-temperature plasma model with a soft component of 3-10 MK and a hard component up to 40 MK. All nondetections belong to the weak-wind class. A group of stars exhibit hybrid properties?flarelike behavior superimposed on a constant component with log LX ~ 32 ergs s-1?which suggest both magnetic activity and wind emission.

X-shooter spectroscopy of young stellar objects: I - Mass accretion rates of low-mass T Tauri stars in \sigma Orionis

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 Orionis cluster. The sample includes eight stars with evidence of disks from Spitzer and four without, 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, 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, we confirm the validity of the correlations given in the literature, with the possible exception of H . 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 often in better agreement 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 non-simultaneous observations of lines and continuum. Three stars in our sample deviate from this behavior, and we discuss them individually.

Evolution of Brown Dwarf Disks: A Spitzer Survey in Upper Scorpius

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.

X-rays from accretion shocks in T Tauri stars: The case of BP Tau

We present an XMM-Newton observation of the classical T Tauri star BP Tau. In the XMM-Newton RGS spectrum the O  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.

A Disk Census for Young Brown Dwarfs

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-band (3.8 μm) disk excess in ~50 spectroscopically confirmed objects near and below the substellar boundary in several young clusters. Our observations, using the ESO Very Large Telescope, Keck I, and the NASA Infrared Telescope Facility, reveal that a significant fraction of brown dwarfs harbor disks at a very young age. Their inner disk lifetimes do not appear to be vastly different from those of disks around T Tauri stars. Our findings are consistent with the hypothesis that substellar objects form via a mechanism similar to solar-mass stars.

On the origin of the X-ray emission from Herbig Ae/Be stars

Context. Herbig Ae/Be stars are fully radiative and not expected to support dynamo action analogous to their convective lower-mass counterparts, the T Tauri stars. Alternative X-ray production mechanisms, related to stellar winds or star-disk magnetospher es have been proposed, but all in all their X-ray emission has remained a mystery. Aims. A study of Herbig Ae/Be stars’ global X-ray properties (such as detection rate, luminosity, temperature, variability) , helps to constrain 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. We performed a systematic search for Chandra archival observations of Herbig Ae/Be stars. The superior spatial resolut ion of this satellite with respect to previous X-ray instrumentation h as allowed us to examine also the possible role of late-type companions in generating the observed X-rays. Results. In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-ray emitting faint companions or other unrelated X-ray bright objects within 10 ′′ . The detection fraction of Herbig Ae/Be stars is 76 %, but lowers to 35 % if all emission is attributed to further known and unresolved companions. The spectral analysis confirms the h igh X-ray temperatures (� 20 MK) and large range of fractional X-ray luminosities (log Lx/L∗) of this class derived from earlier studies of individual ob jects. Conclusions. Radiative winds are ruled out as emission mechanism on basis of the high temperatures. 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), nor from other young late-type stars used for comparison. Therefore, either a similar kind of process takes place on both classes of objects, or there must be as yet undiscovered companion stars.