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New Members of the Scorpius-Centaurus Complex and Ages of Its Sub-regions
We have spectroscopically identified ~100 G-, K-, and M-type members ofthe Scorpius-Centaurus complex. To deduce the age of these young starswe compare their Li ?6708 absorption line strengths against thoseof stars in the TW Hydrae association and ? Pictoris moving group.These line strengths indicate that Sco-Cen stars are younger than ?Pic stars whose ages of ~12 Myr have previously been derived from akinematic traceback analysis. Our derived age, ~10 Myr, for stars in theLower Centaurus Crux and Upper Centaurus Lupus subgroups of ScoCen isyounger than previously published ages based on the moving clustermethod and upper main-sequence fitting. The discrepant ages are likelydue to an incorrect (or lack of) cross-calibration betweenmodel-dependent and model-independent age-dating methods.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| The Spitzer c2d Survey of Weak-line T Tauri Stars. III. The Transition from Primordial Disks to Debris Disks
We present 3.6 to 70 ?m Spitzer photometry of 154 weak-line T Tauristars (WTTSs) in the Chamaeleon, Lupus, Ophiuchus, and Taurus starformation regions, all of which are within 200 pc of the Sun. For acomparative study, we also include 33 classical T Tauri stars which arelocated in the same star-forming regions. Spitzer sensitivities allow usto robustly detect the photosphere in the IRAC bands (3.6 to 8 ?m)and the 24 ?m MIPS band. In the 70 ?m MIPS band, we are able todetect dust emission brighter than roughly 40 times the photosphere.These observations represent the most sensitive WTTSs survey in the mid-to far-infrared to date and reveal the frequency of outer disks (r =3-50 AU) around WTTSs. The 70 ?m photometry for half the c2d WTTSssample (the on-cloud objects), which were not included in the earlierpapers in this series, those of Padgett et al. and Cieza et al., arepresented here for the first time. We find a disk frequency of 19% foron-cloud WTTSs, but just 5% for off-cloud WTTSs, similar to the valuereported in the earlier works. WTTSs exhibit spectral energydistributions that are quite diverse, spanning the range from opticallythick to optically thin disks. Most disks become more tenuous than Ldisk/L * = 2 × 10-3 in 2 Myr andmore tenuous than L disk/L * = 5 ×10-4 in 4 Myr.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| The Lupus Association of Pre-Main-Sequence Stars: Clues to Star Formation Scattered in Space and Time
Kinematical analysis of spectroscopically identified pre-main-sequencestars associated with the Lupus dark cloud reveals a streamlike motionof low internal velocity dispersion (<=1.3 km s-1). Astatistically significant mismatch between the convergent point radialvelocity and the spectroscopic radial velocity from the literatureindicates a moderate degree of expansion. The rate of expansion is toolow to account for the present extent of the association if one assumesthat the spatially dispersed population was formed in the densemolecular cores observed today. Therefore, it is unlikely that theoutlying weak-lined T Tauri members were born in the same star-formingcores as the more compactly located classical T Tauri stars, despite thekinematic integrity of the association. Distances inferred from theclassical moving-cluster method show a large depth of the association(~80 pc) along the line of sight. A color-magnitude diagram of theassociation in the near-IR colors corrected for the distribution ofdistances shows a clear gap separating the older (5-27 Myr) and theyounger (~1 Myr) generations of stars. Half of the identified 1 Myr oldstars lie in the tight group of mostly classical T Tauri starsassociated with the Lupus 3 dark filament. This nest of very young starsappears to be 25 pc farther from the Sun than the center of the greaterLupus association.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| Proper motions of pre-main sequence stars { } in southern star-forming regions
We present proper motion measurements of pre-main sequence (PMS) starsassociated with major star-forming regions of the southern hemisphere(Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis),situated in the galactic longitude range l = 290degr to l = 360degr . Alist of PMS stars as complete as possible was established based on theHerbig and Bell catalogue and many new catalogues like the PDS survey,the catalogue of Herbig Ae/Be stars by Thé et al. (\cite{the}),X-rays surveys, etc. The measurements made use of public material(mainly AC2000 and USNO-A2.0 catalogues) as well as scans of SERC-JSchmidt plates with the MAMA measuring machine (Paris) and Valinhos CCDmeridian circle observations (Brazil). We derived proper motions for 213stars, with an accuracy of 5 to 10 mas/yr depending mainly on thedifference of epochs between the position sources. The maincharacteristics of the sample are discussed. We show that systematicmotions of groups of stars exist, which are not explained by the reflexsolar motion. Based on observations made at Valinhos CCD MeridianCircle. Based on measurements made with MAMA automatic measuringmachine. Table 4 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| High-resolution spectroscopy of ROSAT-discovered weak-line T Tauri stars near Lupus
We present high-resolution optical echelle spectroscopy for a largefraction of the Li-rich late-type stars recently discovered in thevicinity of the Lupus dark clouds. Our results confirm the highLiilambda6708 equivalent widths previously estimated frommedium-resolution spectra, thus adding strength to the conclusion thatthe large majority of these stars are still in the pre-main-sequencephase of their evolution, contrary to claims from other authors thatmany of them might be zero-age main-sequence stars. We present astatistical approach to derive a mean distance for the sample, and findthat it is consistent with, or slightly lower than, the Hipparcosdistance of the Lupus star-forming region. The radial velocitiesmeasured for part of these stars are consistent with those observed forthe Lupus star-forming region, while stars outside the dark clouds showa mean difference of the order of 3kms^-1. The projected rotationalvelocities show a lack of slow rotators, which is interpreted as aconsequence of the X-ray selection of the sample. The Li-rich stars inLupus studied in this work yield a fairly `clean' sample of very youngstars, while in other star-forming regions a larger fraction of olderzero-age main-sequence stars has been found among ROSAT-discoveredLi-rich stars. We argue that this fact reflects the relation of thesestars with the Gould Belt.
| HIPPARCOS results for ROSAT-discovered young stars
Out of ~ 500 Lithium-rich ROSAT counterparts, which were presumed to below-mass pre-main sequence stars, 21 stars have been observed byHIPPARCOS. We study their parallaxes, proper motions, and photometricdata. For 7 out of 10 Taurus and Lupus stars in our sample, propermotions and parallaxes are not inconsistent with membership to theseassociations, while most of the stars in Chamaeleon and Scorpius appearto be young foreground stars. Combined with ground based photometry andspectroscopy, HIPPARCOS parallaxes allow us to place 15 stars on an H-Rdiagram. All these 15 stars lie above the Zero-Age-Main-Sequence andthus are indeed pre-main sequence stars with ages from 1 to 15 Myr. Onlytwo of the stars are located on the Hayashi-tracks, whereas the other 13are post-TTauri stars located on radiative tracks. Although the sampleis admittedly small, containing only 3% of the total sample ofLithium-rich ROSAT counterparts, it does not confirm recent predictionsby other authors: We find no stars in the age range from 20 to 100 Myr.The foreground pre-main sequence stars may have been ejected toward us,or they belong to the Gould Belt system, a plane filled with youngstars.
| New "weak-line"--T Tauri stars in Lupus
We present first results obtained by a survey of the Lupus star formingregion in search of new T Tauri stars. This study has been performed onthe basis of deep pointed ROSAT observations in the Lupus dark clouds aswell as data from the ROSAT All-Sky-Survey in the surrounding, lessobscured regions. Our survey covers an area of about 230 square degrees,located between 15^h,6^m and 16^h,24^m$ in right ascension and between-47^\circ and -32^\circ in declination. Identification of ROSATAll-Sky-Survey sources in this area by means of optical spectroscopyrevealed 89 T Tauri stars, 86 of them "weak-line" T Tauri stars (WTTS)not known from previous studies of this region. Our pointed ROSATobservations led to the identification of 47 more T Tauri stars, givinga total of 136 new T Tauri stars. The large area of our study, ascompared with previous works, allows us to study the spatialdistribution of WTTS in this star forming region on a large scale. Wefind the new WTTS to be distributed over the whole area of our survey,indicating that their spatial distribution might extend well beyond ourstudy area. Contrary to the Lupus T Tauri stars known prior to thisstudy, the WTTS discovered by the ROSAT All-Sky-Survey are not clusteredin the regions of highest extinction, i.e. the dark clouds. Based onobservations collected at European Southern Observatory, La Silla, Chile(observing proposals ESO Nos. 49.7-0010, 50.7-0109, 51.7-0106,51.7-0029). Tables 5--12 are only available in electronic form at theCDS via anonymous ftp 130.79.128.5 or on www athttp://cdsweb.u-strasbg.fr/abstract.html.
| The T Tauri star population in the Lupus star forming region.
In a recent study, some 130 new weak-line T Tauri stars (WTTS) have beendiscovered in the Lupus star forming region (SFR). Some of these starsare seen projected onto regions of high obscuration, while others arelocated far from the Lupus dark clouds. In this paper we presentphotometric observations of a large sample of these WTTS. We estimateeffective temperatures and luminosities for the stars observed, andderive masses and ages by comparison with theoretical evolutionarytracks. The mean age of WTTS seen in projection against the dark cloudsis found to be lower than the mean age of WTTS discovered far fromregions of high obscuration, and yet higher than the mean age of theclassical T Tauri stars (CTTS) in Lupus. Moreover, while the CTTS inLupus show an unusual predominance of very low-mass stars, the WTTSpopulation in Lupus contains many stars with comparatively highermasses. Correlations between the X-ray emission and other stellarparameters, like bolometric luminosity, radius, mass, and age, arestudied, and the results are discussed.
| Metal-Deficient Giants in the Galactic Field - Catalogue and Some Physical Parameters
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