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The short period multiplicity among T Tauri stars
We present the results of high-resolution spectroscopic observationscarried out over three years aimed at estimating the short-period(P_orb<100 days) binary frequency of a sample of T Tauri stars inOph-Sco, Cha, Lup, CrA star forming regions (SFRs), already observedwith high angular resolution techniques by Ghez et al.(\cite{Ghezetal93}) and by Ghez et al. (\cite{Ghezetal97}) to detectwider components. When combining all four SFRs, the short-period binaryfrequency is indistinguishable from that found by Duquennoy & Mayor(\cite{DuqMay91}) for the solar-type field stars which is alsoconsistent with the previous result obtained by Mathieu(\cite{Mathieu92}, \cite{Mathieu94}). When Oph-Sco is analyzedseparately, it seems that there is an excess of short-period binaries ofa factor 2-2.5. On the contrary, short-period binary systems seem to beabsent in the sample containing stars in Cha/Lup/CrA. Such a trend wasequally found by Mathieu (\cite{Mathieu92}) in Taurus. An excess ofspectroscopic systems among the components of visual multiple systems isalso observed.Based on observations collected with the Swiss Euler Telescope and 1.5-mESO, proposal 63.I-0112.
| 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
| Starlight polarization and CO observations towards the Lupus clouds
We performed an observational study of the dark filaments Lupus 1 andLupus 4 using both polarimetric observations of 190 stars and a sampleof 72 ^12CO profiles towards these clouds. We have estimated lowerlimits to the distances of Lupus 1 and Lupus 4 (>~ 140 and >~ 125pc, respectively). The observational strategy of the survey allows us tocompare the projected magnetic field in an extended area around eachcloud with the magnetic field direction observed to prevail along theclouds. Lupus 4 could have collapsed along the magnetic field lines,while in Lupus 1 the magnetic field appears to be less ordered, havingthe major axis of the filaments parallel to the large-scale projectedmagnetic field. These differences would imply that both filaments havedifferent pattern evolutions. From the CO observations we have probedthe velocity fields of the filaments and the spatial extension of themolecular gas with respect to the dust.
| 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 Multiplicity of Pre-Main-Sequence Stars in Southern Star-forming Regions
High-resolution studies of young stars in the star-forming regions ofTaurus and Ophiuchus have revealed a large population of multiple starsystems. To test how applicable this earlier result is for otherstar-forming regions, we have carried out a K-band (2.2 mu m)multiplicity survey of pre--main-sequence stars located in the darkcloud complexes Chameleon, Lupus, and Corona Australis. This survey,which was conducted with both speckle- and direct-imaging techniques,covers a binary star separation range of 0."1--12" (15--1800 AU) andidentifies 25 companion stars of which nine are new detections. Thecompanion star fraction over the separation range covered by this surveyis estimated to be 0.52 +/- 0.11, in agreement with Taurus (0.58 +/-0.08) and Ophiuchus (0.50 +/- 0.12). A comparison of the direct-imagingportion of this survey with Reipurth & Zinnecker's opticalmultiplicity study reveals that 4% of the overlap sample have "infraredcompanions," companions too red to be detected at optical wavelengths.This suggests that infrared surveys will systematically measure aslightly higher companion star fraction compared with optical surveys.The result of combining all K-band surveys of dark cloud complexes,which cover the separation range 15--1800 AU, shows a factor of 2 excessof the companion star fraction for young stars compared with that forthe solar-type stars in the solar neighborhood (0.54 +/- 0.06 vs. 0.26+/- 0.04).
| Radial and rotational velocities of young stars in Chamaeleon and Lupus.
We present a kinematic study of 26 T Tauri stars in the Chamaeleon andLupus molecular clouds, based on high resolution CASPEC spectra andCORAVEL radial velocity scanner observations. After discarding starswith high rotational velocities, noisy spectra or apparently variablevelocities, we find a velocity dispersion of 0.9+/-0.3km/s for theremaining 10 stars of the Cha I T Tauri association. We also observe theJ=1-0 line of ^13^CO towards our target stars in Cha I, and find nosignificant gas motions to within 0.3km/s, corresponding to the accuracyof the measurements. We derive a value of 0.5+/-0.4km/s for thedifference between the mean stellar radial velocity and the mean gasvelocity. The stars and the gas in Cha I are thus still closely linkedkinematically. In the Lupus 3 clouds our sample of stars is smaller, andwe merely note that the stars observed divide into two kinematicallydistinct subgroups, one of which belongs to the background Carina arm.Clearly a larger number of measurements would be desirable. We argue,however, that the accuracy of the stellar radial velocities may bedifficult to improve because of the presence of undetected long-periodspectroscopic binaries and large star spots. In addition, thedetermination of the gas motions is limited by complex line profiles dueto cloud structure along the line-of-sight.
| The stellar population of the Lupus clouds
We present photometric and spectroscopic observations of the H alphaemission stars in the Lupus dark cloud complex. We estimate theeffective temperatures of the stars from their spectral types andcalculate the reddening towards each object from the (R-I) colors. Fromthese data, we derive mass and age distributions for the Lupus starsusing a new set of pre-main sequence evolutionar tracks. We compare theresults for the Lupus stars with those for a similar population of youngstellar objects in Taurus-Auriga and Chamaeleon and with the initialmass function for field stars in the solar neighborhood. From the H-Rdiagrams, Lupus appears to contain older stars than Taurus. The Lupusdark clouds form a greater proportion of low mass stars than the Tauruscomplex. Also, the proportion of low mass stars in Lupus is higher thanthat predicted by the Miller-Scalo initial mass function, and the lowestmass stars in Lupus are less active than similar T Tauri stars in otherregions.
| A large, complete, volume-limited sample of G-type dwarfs. I. Completion of Stroemgren UVBY photometry
Four-colour photometry of potential dwarf stars of types G0 to K2,selected from the Michigan Spectral Catalogues (Vol. 1-3), has beencarried out. The results are presented in a catalogue containing 4247uvby observations of 3900 stars, all south of δ = -26deg. Theoverall internal rms errors of one observation (transformed to thestandard system) of a program star in the interval 8.5 < V < 10.5are 0.0044, 0.0021, 0.0039, and 0.0059, respectively, in V, b-y, m_1_ ,and c_1_. The purpose of the catalogue, combined with earliercatalogues, is to allow selection of a large, complete, volume-limitedsample of G- and K-type dwarfs, investigate their metallicitydistribution, and compare it to predictions of various models ofgalactic chemical evolution. Future papers in this series will discussthese subjects.
| A survey of southern dark clouds for Herbig-Haro objects and H-alpha emission stars
Abstract image available at:http://adsabs.harvard.edu/abs/1977ApJS...35..161S
| On faint H-alpha emission stars in Lupus and Scorpius.
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