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Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| Photometry of 39 PMS Variables in the Taurus-Auriga Region
Thirty nine PMS variable stars were observed in the direction of theTaurus-Auriga star-forming region.
| A kinematic study of the Taurus-Auriga T association
Aims.This is the first paper in a series dedicated to investigating thekinematic properties of nearby associations of young stellar objects.Here we study the Taurus-Auriga association, with the primary objectiveof deriving kinematic parallaxes for individual members of this low-massstar-forming region. Methods: .We took advantage of a recentlypublished catalog of proper motions for pre-main sequence stars, whichwe supplemented with radial velocities from various sources found in theCDS databases. We searched for stars of the Taurus-Auriga region thatshare the same space velocity, using a modified convergent point methodthat we tested with extensive Monte Carlo simulations. Results:.Among the sample of 217 Taurus-Auriga stars with known proper motions,we identify 94 pre-main sequence stars that are probable members of thesame moving group and several additional candidates whose pre-mainsequence evolutionary status needs to be confirmed. We derive individualparallaxes for the 67 moving group members with known radial velocitiesand give tentative parallaxes for other members based on the averagespatial velocity of the group. The Hertzsprung-Russell diagram for themoving group members and a discussion of their masses and ages arepresented in a companion paper.
| 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.
| The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga
We present an astrometric study of the candidates of T Tauri stars (TTS)and non-TTS X-ray sources around Tau-Aur, based on the HipparcosCatalogue and the ACT Reference Catalogue. The ROSAT selected X-raysources are found to be a mixed population. A few of them areassociated with the Tau-Aur or Orion Star Forming Regions (SFR). Some,with distances similar to that of Tau-Aur but with discrepant propermotions, are probable or sure Pleiades super-cluster members or otherlate type young active stars with unresolved nature, more likely tooriginate in rapidly moving cloudlets, or else having originated fromdifferent sites other than Tau-Aur and moved to the present locations. Agood many of the non-TTS X-ray sources are considered as Hyades clustermembers. Some TTS candidates could be foreground pre-main sequencestars or actually young dwarfs not yet depleted of their Lithium. Underthe hypothesis that the sources we studied are representative of theROSAT selected TTS candidates discovered in the outskirts of the Tau-Aurregion, we conclude that only up to one third of the weak-line TTScandidates could be expected to be physically associated with theTau-Aur association. Along with the parallax and proper motion analysisof the non-TTS X-ray sources around the Tau-Aur SFR, our result suggeststhat the vast majority of the young active X-ray sources within anangular diameter of about 30 ° of the Tau-Aur SFR, belong to fourmain subgroups that are spatially separate.
| Comments on Inferences of Star Formation Histories and Birth Lines
Palla & Stahler have recently argued that star formation in Taurusand other nearby molecular clouds extends over a period of at least 10Myr, implying quasi-static cloud evolution and star formation. Theirconclusions contradict other recent results indicating that molecularclouds are transient objects and star formation proceeds rapidly. ThePalla & Stahler picture implies that most molecular clouds shouldhave extremely low rates of star formation and that in such inactivestages, the stellar initial mass function (IMF) should be stronglyskewed toward producing stars with masses >~1Msolarneither prediction is supported by observations. I show that the Palla& Stahler conclusions for Taurus depend almost entirely on a smallnumber of stars with masses >~1Msolar the lower mass starsshow no evidence for such an extended period of star formation. Ifurther show that most of the stars apparently older than 10 Myr in thedirection of Taurus are probably foreground nonmembers. I also presentbirth line calculations that support the idea that the ages of the starswith masses >~1Msolar have been systematicallyoverestimated because birth line age corrections have beenunderestimated; such birth lines would eliminate the need to postulateskewed IMFs. The simplest and most robust explanation of currentobservations characterizing the vast majority of young stars inmolecular clouds is that cloud and star formation is rapid and dynamic.
| Star Formation in Space and Time: Taurus-Auriga
To understand the formation of stellar groups, one must first documentcarefully the birth pattern within real clusters and associations. Inthis study of Taurus-Auriga, we combine pre-main-sequence ages from ourown evolutionary tracks with stellar positions from observationalsurveys. Aided by the extensive millimeter data on the molecular clouds,we develop a picture of the region's history. Star formation began, at arelatively low level and in a spatially diffuse manner, at least107 yr in the past. Within the last few million years, newstars have been produced at an accelerating rate, almost exclusivelywithin a confined group of striated cloud filaments. The gas both insideand around the filaments appears to be in force balance. Thus, theappearance of the filaments is due to global, quasi-static contractionof the parent cloud material. Gravity drives this contraction and shockdissipation mediates it, but the internal motion of the gas does notappear to be turbulent. The accelerating nature of recent star formationmeans that the condensation of cloud cores is a threshold phenomenon,requiring a minimum background density. Other, nearby cloud regions,including Lupus and Chamaeleon, contain some locales that have attainedthis density, and others that have not. In the latter, we find extensiveand sometimes massive molecular gas that is still devoid of young stars.
| Photometric observations of weak-line T Tauri stars . II. WTTS in Taurus-Auriga, Orion and Scorpius OB2-2
We present uvby-$beta a photometry of 116 X-ray flux-selected activestars in the directions of the Orion (40), Taurus-Auriga (58) andScorpius OB2-2 (18) star forming regions. Additionally, we give near IRJHK photometry of 20 active stars in the Taurus-Auriga direction. Theprogram stars were selected from the R_\odotsat All Sky Survey andEINSTEIN X-ray surveys and are spectroscopically confirmed weak-line TTauri stars and weak-line T Tauri star candidates. The photometryconfirms the young nature of the program stars and also indicates that asignificant fraction of the sample could be foreground objects. The datagiven here probably represent the largest homogeneous uvby-beta a$photometric sample of new WTTS and WTTS candidates. Many objects in thesample are observed photometrically for the first time. Based onobservations collected at the Observatorio Astronómico Nacionalin Sierra San Pedro Mártir, Baja California, México.Tables 1-4 are also 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
| A study of Li-rich stars discovered by ROSAT in Taurus-Auriga
In recent years, large numbers of lithium-rich stars were discoverednear several nearby star forming regions (SFRs). We present a detailedstudy of those stars discovered in and near the central region of theTaurus-Auriga T Tauri association, based on high-resolution echellespectroscopy and proper motion data. We find that about 60 per cent ofour sample can be regarded as pre-main sequence (PMS) stars, while theremaining stars likely are foreground zero-age main sequence (ZAMS)stars. We conclude that the PMS stars are likely associated with theTaurus-Auriga SFR, while the ZAMS stars may represent a population ofsomewhat older Gould Belt stars. The fraction of ZAMS stars in theTaurus-Auriga sample studied in this work is larger than in a similarsample in the Lupus SFR, and we argue that this may be explained by thespatial structure of the Gould Belt and the Sun's location within it.Based on observations obtained at Observatoire de Haute Provence. Someof the observations reported here were obtained with the Multiple MirrorTelescope, a joint facility of the Smithsonian Institution and theUniversity of Arizona.
| Revisiting Hipparcos data for pre-main sequence stars
We cross-correlate the Herbig & Bell and Hipparcos Catalogues inorder to extract the results for young stellar objects (YSOs). Wecompare the distances of individual young stars and the distance oftheir presumably associated molecular clouds, taking into accountpost-Hipparcos distances to the relevant associations and usingHipparcos intermediate astrometric data to derive new parallaxes of thepre-main sequence stars based on their grouping. We confirm that YSOsare located in their associated clouds, as anticipated by a large bodyof work, and discuss reasons which make the individual parallaxes ofsome YSOs doubtful. We find in particular that the distance of TaurusYSOs as a group is entirely consistent with the molecular clouddistance, although Hipparcos distances of some faint Taurus-Auriga starsmust be viewed with caution. We then improve some of the solutions forthe binary and multiple pre-main sequence stars. In particular, weconfirm three new astrometric young binaries discovered by Hipparcos:RY Tau, UX Ori, and IXOph. Based on observations made with the ESA Hipparcosastrometry satellite
| Multiplicity of T Tauri stars in Taurus after ROSAT
We surveyed a sample of 75 T Tauri stars in the Taurus star formingregion for companions. These stars were discovered with the help ofROSAT. The separation range covered is 0.13'' to 13'', where the lowerlimit is given by the diffraction limit of the telescope and the upperlimit by confusion with background stars. Combined with the results ofthe preceding survey by Leinert et al. (\cite{Leinert93}), we now havesurveyed a sample of 178 young stars in Taurus, 63 classical, 106weak-line, and 9 unclassified T Tauri stars. Within this sample, we find68 binaries, 9 triples, and 3 quadruples. After corrections to accountfor confusion with background stars and for a bias induced through X-rayselection, we count 74 binaries or multiples with a total of 85companions in 174 systems. This corresponds to a degree of multiplicity(number of binaries or multiples divided by number of systems) of(42.5+/- 4.9)%, or to a duplicity, measured by the number of companionsper system, of (48.9+/- 5.3)%, which is higher by a factor of (1.93+/-0.26) compared to solar-type main-sequence stars. We find no differencein duplicity between classical and weak-line T Tauri stars. There is adifference between close and wide pairs in the sense that close pairshave a flat distribution of flux ratios, while the flux ratios of widepairs are peaked towards small values.
| 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.
| Radio Emission from ROSAT-discovered Young Stars in and around Taurus-Auriga
An 8.4 GHz VLA survey of 91 recently discovered lithium-rich late-typestars from the ROSAT All-Sky Survey and pointed observations ispresented. These objects lie in the vicinity of the Taurus-Aurigastar-forming region (d ~= 140 pc); however, some are dispersed nearly 30deg from known active star-forming cloud cores. This sample represents aspatially complete, flux-limited population of X-ray-bright young starsboth within and away from the primary Tau-Aur stellar nurseries. Of the91 sources, 29 are detected in this radio survey with a sensitivitylimit of ~0.15 mJy. If they are at the distance of the star-formingclouds, we find that 32% of widely distributed young stars with LX >=5 x 1028 ergs s-1 have radio luminosity densities in excess of 3.5 x1015 ergs s-1 Hz-1. This detection rate, the ranges of radio and X-rayluminosities, and the LR/LX ratios are consistent with known youngweak-lined T Tauri stars (ages ~106 yr) that reside within the Taurusmolecular clouds, but they are considerably higher than a zero-agemain-sequence population such as the Pleiades (age ~=7 x 107 yr). Theradio properties thus support the pre-main-sequence classification ofthe stars. They fitted well among other active young stars on theempirical LR versus LX diagram, implying that solar-type gyrosynchrotronactivity is the radio emission mechanism.
| New proper motions of pre-main sequence stars in Taurus-Auriga
We present proper motions of 72 T Tauri stars located in the centralregion of Taurus-Auriga (Tau-Aur). These proper motions are taken from anew proper motion catalogue called STARNET. Our sample comprises 17classical T Tauri stars (CTTS) and 55 weak-line T Tauri stars (WTTS),most of the latter discovered by ROSAT. 53 stars had no proper motionmeasurement before. Kinematically, 62 of these stars are members of theassociation. A velocity dispersion of less than 2-3km/s is found whichis dominated by the errors of the proper motions. This velocitydispersion correlates with a spread in distances. Furthermore we presentproper motions of 58 stars located in a region just south of the Taurusmolecular clouds and compare the kinematics of the youngest stars inthis sample (younger than 3.5x10^7^yrs) with the kinematics of thepre-main sequence stars (PMS) in the Taurus-Auriga association. From acomparison of the space velocities we find that the stars in the centralregion of Tau-Aur are kinematically different from the stars in thesouthern part. Among the stars with large proper motions far off theTaurus mean motion we find 2 Pleiades candidates and 7 possible Pleiadesrunaway stars.
| New weak-line T Tauri stars in Taurus-Auriga.
On the basis of the ROSAT All-Sky-Survey, a study of the Taurus-Aurigastar forming region has been performed in order to search for hithertoundiscovered TTauri stars. Our study covers an area of about 280 squaredegrees, located between 4^h^ and 5^h^ in right ascension and between15deg and 34deg in declination. Identification of ROSAT All-Sky Surveysources in this area by means of optical spectroscopy revealed 2 newclassical T Tauri stars (CTTS) and 66 new weak-line-T Tauri stars (WTTS)with Wlambda_(Hα)<=10A. Additional pointed ROSATobservations led to the identification of 6 more WTTS and 2 CTTS, givinga total of 76 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. We find the WTTS ofour survey to be distributed over the whole region investigated. Thereis a noticeable decline of the surface density from south to northwithin our study area, but the spatial distribution extends mostprobably beyond our study region. No clustering towards the populationof TTauri stars known prior to ROSAT in Taurus-Auriga could be observed.We suggest that the WTTS found in our study might in part be somewhatolder than the previously known TTauri stars in Taurus-Auriga, and thattheir broad spatial distribution is due to the typical velocitydispersion of a few km/s measured for Taurus TTauri stars, in which casefor some of our WTTS an age on the order of 10^7^years would be requiredfor reaching the observed distances from the Taurus dark clouds. Weestimate a WTTS/CTTS ratio of about 6 within our study area, butconclude that because of the different spatial distribution of WTTS andCTTS this ratio will be most probably significantly larger for a moreextended area.
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
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