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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.
| Multiplicity of X-Ray-selected T Tauri Stars in Chamaeleon
We report on a multiplicity survey of a sample of X-ray-selected youngstars in the Chamaeleon association. We used speckle interferometry anddirect imaging to find companions in the separation range 0.13"-6".After correction for chance alignment with background stars, we find amultiplicity (number of binaries or multiples divided by number ofsystems) of 14.0%+/-4.3% and a companion-star frequency (number ofcompanions divided by number of systems) of 14.7%+/-5.1%. Compared withsolar-type main-sequence stars, the companion-star frequency is lower bya factor of 0.61+/-0.27. This is remarkably different from the highmultiplicity found in the Taurus-Auriga star-forming region and for TTauri stars in Chamaeleon known before ROSAT. We find only a fewbinaries with projected separations of more than 70 AU, also in contrastto the results for stars known before ROSAT. This indicates that theX-ray-selected stars belong to a different population than the starsknown before ROSAT, a hypothesis further supported by their Hipparcosdistances and proper motions. Based on observations obtained at theEuropean Southern Observatory, La Silla, proposal 56.E-0197.
| 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
| CCD photometry and astrometry of visual double and multiple stars of the HIPPARCOS Catalogue. II. CCD photometry and differential astrometry of 288 southern ``Intermediate'' systems
We present photometric and astrometric data of about 280 visual doublestars of the ``intermediate'' class, i.e. with angular separationsmainly in the range 2arcsec < rho < 12arcsec . The observationshave been obtained in 1991-92 with a CCD camera attached to the 91 cmDutch telescope at ESO La Silla, Chile. Differential magnitudes of thedouble star components as well as magnitudes and colour indices of theindividual components have been determined in the Cousins V and Ipassbands with an internal error of about 0.005 mag and an externalaccuracy of less than 0.03 mag. In addition, angular separations havebeen secured to an internal accuracy of 0.004arcsec and position anglesto about 0.05degr. Tables 1 to 4 are only available in electronic format the CDS via ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html. Based on observationscollected at the European Southern Observatory, La Silla, Chile.
| Proper motions of faint ROSAT WTT stars in the Chamaeleon region
We present proper motions of 59 stars of the ROSAT All-Sky Survey (RASS)located in direction of the Chamaeleon star forming region (SFR) in themagnitude range B=5.1 - 17 mag. Proper motions of the fainter stars werenewly derived utilizing survey Schmidt plates from the GSC II platearchive and from a set of special plates taken with the ESO Schmidttelescope. The vector point diagram (VPD) indicates that the certifiedWTT stars cluster away from the region occupied by the brighterpre-main-sequence stars (PMS) in Cha I. The distance to this newassociation is estimated at ~ 100 pc, sensibly smaller than the 150 pcgenerally assumed for the SFR. This yields an upper limit of 2 km s(-1)for the velocity dispersion of this new kinematic group. The de-reddenedCM diagram of the group members suggest the WTT stars are still PMSobjects, but older (3-30 Myr) and less massive than previousdeterminations. These revised age estimates, the newly derived grouppeculiar velocity, and current distance estimates to the Cha I/II/IIIcomplex would favour in-situ formation against that predicted by highvelocity cloud models. Finally, based on a redetermination of thepeculiar motions of stars and gas, we speculate that the whole SFRoriginated from the local Orion spur as a result of more classicalmechanisms like interactions with the spiral arms. Based on observationscollected at the European Southern Observatory (Chile) and on data fromthe Hipparcos astrometry satellite. Table~1 is available only inelectronic form at the CDS via anonymous ftp at 130.79.128.5.
| Kinematics of T Tauri stars in Chamaeleon
We study the kinematics of T Tauri stars (TTS) located in the cores ofthe Chamaeleon clouds as well as far off these clouds. Our samplecomprises 2 early type stars known to be related to Cha i, 6 classical(CTTS) and 6 weak-line T Tauri stars (WTTS) known before the ROSATmission, and 8 bona-fide pre-main sequence (PMS) stars as well as 23presumably older stars discovered with ROSAT (Alcalaet al. 1995; Covinoet al. 1997). Altogether we present proper motions for 45 stars, takenfrom the Hipparcos, ACT and STARNET catalogues. For 12 stars of oursample parallaxes measured by Hipparcos are available, and we use themto derive constraints on the distance distribution of the other stars inour sample. Our analysis of the proper motions allows us to divide thesample into several subgroups. We analyse the motions of the stars inconnection with different star formation scenarios and find themconsistent with both the high velocity cloud (HVC) impact model (Lepine& Duvert 1994) and the cloudlet model (Feigelson 1996), whereas thedata seem to be inconsistent with any kind of a dynamical ejectionmodel.
| 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.
| A study of the Chamaeleon star-forming region from the ROSAT all-sky survey. III. High resolution spectroscopic study
We present the results of a high-resolution spectroscopic study on some70 stars, discovered recently on the basis of the ROSAT all-sky surveyspread over a wide area in the Chamaeleon star forming region andclassified as new weak-line T Tauri stars. We refine the previousspectral type classification, based on low-resolution spectra, andcharacterize each star in the sample according to the Hα lineprofile. We use the strength of Li I 6708, compared to Pleiades stars ofthe same spectral type, as a youth discriminator in order to recognizebona-fide pre-main sequence stars. According to the adopted ``lithiumcriterion'', more than 50% of the stars in our sample are confirmed tobe truly young, PMS stars (most having age less than 5x 10(6) yr), whilethe remaining part seems mostly composed by active, young, foregroundmain-sequence stars (possibly Pleiades-like), which contaminate theoriginal sample. We confirm the existence of some very young stars farfrom the main Chamaeleon clouds, while we do not find clear evidence forthe presence of post-T Tauri stars in our sample. We find that 5 starsin the sample are spectroscopic binaries and 1 is a spectroscopic triplesystem. We derive radial and rotational velocities for all the stars insample and analyse their distributions for different spectral typeintervals. The radial velocity distribution shows a clear peak at about15 km s(-1) , which coincides with the radial velocity of stars and gasin the Cha I cloud. However, the velocity dispersion of the weak-line TTauri stars appears much broader and, possibly, a second peak is presentaround 16-18 km s(-1) . A clear segregation in radial velocity isobserved between the strong-lithium and the weak-lithium stars, with theformer showing radial velocities which, in most cases, fall in theinterval 12
| Spectrophotometry of flares and short time scale variations in weak line, and classical T Tauri stars in Chamaeleon.
Results are presented from a monitoring program looking for variationsof the equivalent width and flux of Hα, and of the continuum flux,in 18 classical and 18 weak line T Tauri stars, and one Herbig Ae/Bestar, on time scales down to 5 minutes. The stars were observedsimultaiously for 14 hours using the multiobject-spectrograph FLAIR onthe UK Schmidt Telescope. The campaign turned up two events in weak lineT Tauri stars. Both events show the characteristics of flares: Theincrease of the Hα emission is faster than the decline, and theincrease of the emission is much larger in Hα than in thecontinuum. The total energies radiated in Hα are2.0+/-0.7x10^33^erg and >=6+/-2x10^32^ erg, or roughly 200 to 700times larger than the largest flares on the Sun. The spectrum of one ofthese events shows a component of Hα which is blue-shifted byabout 600km.s^-1^. We interpret this component as an indication of massloss, and estimate, if the event is a typical one, that the mass lossrate due to flares would be about 10^-13^Msun_/yr for a weakline T Tauri star. We derive lower limits to the magnetic flux densityof between 10 and 200G. Although a number of events have been observedin the classical T Tauri stars as well, none of these has shown thestrong increase within minutes and decrease in an hour characteristic ofa flare. We interpret this result as being due to either an opticalthickness effect in Hα, or to the absence of optical flares inclassical T Tauri stars, or simply due to failing to catch any flares inclassical T Tauri stars.
| Multiplicity among T Tauri stars in OB and T associations. Implications for binary star formation.
We present first results of a survey for companions among X-ray selectedpre-main sequence stars, most of them being weak-line T Tauri stars(WTTS). These T Tauri stars have been identified in the course ofoptical follow-up observations of sources from the ROSAT All Sky Surveyassociated with star forming regions. The areas surveyed include the Tassociations of Chamaeleon and Lupus as well as Upper Scorpius, thelatter being part of the Scorpius Centaurus OB association (Sco OB 2).Using SUSI at the NTT under subarcsec seeing conditions we observed 195T Tauri stars through a 1μm ("Z") filter and identified companions to31 of them (among these 12 subarcsec binaries). Based on statisticalarguments we conclude that almost all of them are indeed physical (i.e.gravitationally bound) binary or multiple systems. For 10 systemslocated in Upper Scorpius and Lupus, we additionally obtained spatiallyresolved near-infrared photometry in the J, H, and K bands with the MPIA2.2m telescope at ESO, La Silla. The near-infrared colours of thesecondaries are consistent with those of dwarfs and are clearly distinctfrom those of late type giant stars. Based on astrometric measurementsof some binaries we show that the components of these binaries arecommon proper motion pairs, very likely in a gravitationally bound orbitaround each other. We find that the overall binary frequency among TTauri stars in a range of separations between 120 and 1800 AU is inagreement with the binary frequency observed among main sequence starsin the solar neighbourhood. However, we note that within individualregions the spatial distribution of binaries - within a distinct rangeof separation - is non-uniform. In particular, in Upper Scorpius, WTTSin the vicinity of early type stars seem to be almost devoid of multiplesystems, whereas in another area in Upper Scorpius half of all WTTS havea companion in a range of separation between 0.7" and 3.0." Furthermore,we find no preponderance of systems with large brightness differencesbetween primary and companion stars (median {DELTA}Z=1.0mag ...1.5).magWe conclude that binarity is established very early in stellarevolution, that the orbital parameters of wide binaries (a>=120AU)remain virtually unchanged during their pre-main sequence evolution, andthat these wide binaries were formed either through collisionalfragmentation or fragmentation of rotating filaments.
| A study of the Chamaeleon star forming region from the ROSAT all-sky survey. I. X-ray observations and optical identifications.
We present the observations of the ROSAT all-sky survey (RASS) in thedirection of the Chamaeleon cloud complex, as well as the spectroscopicidentifications of the detected X-ray sources. The main purpose of thisidentification program was the search for low mass pre-main sequencestars. Sixteen previously known PMS stars were detected with highconfidence by ROSAT. Eight are classical T Tauri stars and eight areweak-line T Tauri stars, Seventy-seven new weak-line T Tauri stars wereidentified on the basis of the presence of strong Li λ 6707absorption, spectral type later than F0 and chromospheric emission. Wegive coordinates and count rates of the X-ray sources, and presentoptical spectra and finding charts for the sources identified opticallyas new pre-main sequence stars. Optical UBV(RI)_c_ and near-infraredJHKLM photometry for this sample of stars is also provided. In addition,6 new dKe-dMe candidates are found among the RASS sources.
| A southern objective prism survey.
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