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Multiplicity of young stars in and around R Coronae Australis
Context: In star-forming regions like Taurus-Auriga, it has been foundthat most young stars are born as multiples, which theories of starformation should definitely take into account. The R CrA star-formingregion has a small dark cloud with quite a number of protostars, T Tauristars, and some Herbig Ae/Be stars, plus a number of weak-line T Tauristars around the cloud found by ROSAT follow-up observations. Aims: We would like to detect multiples among the young stars in andaround the R CrA cloud in order to investigate multiplicity in thisregion. Methods: We performed interferometric and imagingobservations with the speckle camera SHARP I at the ESO 3.5 m NTT andadaptive optics observation with ADONIS at the ESO 3.6 m telescope, allin the near-infrared bands JHK obtained in the years 1995, 2000, and2001. Results: We found 13 new binaries among the young stars inCrA between 0.13 arcsec (the diffraction limit) and 6 arcsec (set as anupper separation limit to avoid contamination by chance alignments).While most multiples in CrA are binaries, there are also one quadruple(TY CrA), and one triple (HR 7170) which may form a quintuple togetherwith the binary HR 7169. One of the newly detected companions with alarge magnitude difference found near the M3-5 type T Tauri star [MR 81]H? 17 could be a brown dwarf or an infrared companion with anedge-on disk. Among seven Herbig Ae/Be stars in CrA, six are multiple. Conclusions: The multiplicity frequency in CrA is as high as insimilar star forming regions. By comparing with the period distributionof main-sequence stars and extrapolating to separations not probed inthis survey, we conclude that the companion-star frequency is(95±23)%; i.e. the average number of companions per primary is0.95.Based on observations obtained at the European Southern Observatory, LaSilla, proposal numbers 55.E-0968, 65.I-0086, and 67.C-0213.
| On the kinematic evolution of young local associations and the Scorpius-Centaurus complex
Context: Over the last decade, several groups of young (mainly low-mass)stars have been discovered in the solar neighbourhood (closer than ~100pc), thanks to cross-correlation between X-ray, optical spectroscopy andkinematic data. These young local associations - including an importantfraction whose members are Hipparcos stars - offer insights into thestar formation process in low-density environments, shed light on thesubstellar domain, and could have played an important role in the recenthistory of the local interstellar medium. Aims: To study the kinematicevolution of young local associations and their relation to other youngstellar groups and structures in the local interstellar medium, thuscasting new light on recent star formation processes in the solarneighbourhood. Methods: We compiled the data published in theliterature for young local associations. Using a realistic Galacticpotential we integrated the orbits for these associations and theSco-Cen complex back in time. Results: Combining these data with thespatial structure of the Local Bubble and the spiral structure of theGalaxy, we propose a recent history of star formation in the solarneighbourhood. We suggest that both the Sco-Cen complex and young localassociations originated as a result of the impact of the inner spiralarm shock wave against a giant molecular cloud. The core of the giantmolecular cloud formed the Sco-Cen complex, and some small cloudlets ina halo around the giant molecular cloud formed young local associationsseveral million years later. We also propose a supernova in young localassociations a few million years ago as the most likely candidate tohave reheated the Local Bubble to its present temperature.
| 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.
| Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud
We present the ROSAT All-Sky Survey data in a 126 deg2 areain and around the CrA star forming region. With low-resolutionspectroscopy of unidentified ROSAT sources we could find 19 new pre-mainsequence stars, two of which are classical T Tauri stars, the othersbeing weak-lined. The spectral types of these new T Tauri stars rangefrom F7 to M6. The two new classical T Tauri stars are located towardstwo small cloud-lets outside of the main CrA cloud. They appear to be ~10 Myrs old, by comparing their location in the H-R diagram withisochrones for an assumed distance of 130 pc, the distance of the mainCrA dark cloud. The new off-cloud weak-line T Tauri stars may haveformed in similar cloudlets, which have dispersed recently.High-resolution spectra of our new T Tauri stars show that they havesignificantly more lithium absorption than zero-age main-sequence starsof the same spectral type, so that they are indeed young. From thosespectra we also obtained rotational and radial velocities. For somestars we found the proper motion in published catalogs. The directionand velocity of the 3D space motion - south relative to the galaticplane - of the CrA T Tauri stars is consistent with the dark cloud beingformed originally by a high-velocity cloud impact onto the galacticplane, which triggered the star formation in CrA. We also present VRIJHKphotometry for most of the new T Tauri stars to derive theirluminosities, ages, and masses. Partly based on observations collectedat the 1.52 m and 3.5 m telescopes of the European Southern Observatory,Chile, in programs 55.E-0549, 57.E-0646, and 63.L-0023, and onobservations collected at the 0.9 m, 1.5 m, and 4.0 m CTIO telescope.
| 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
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Comparison of Star Formation in Five Nearby Molecular Clouds
We have calculated bolometric temperature (Tbol) and bolometricluminosity (Lbol) for 383 young stellar objects (YSOs) in five molecularclouds within 200 pc in Corona Australis (CrA), Ophiuchus (Oph), Taurus(Tau), Chamaeleon (Cha), and Lupus (Lup). We used Tbol, Lbol, andbolometric luminosity-temperature (BLT) diagrams to characterize andcompare the overall-star-formation activity of the clouds on aself-consistent basis. The main results are the following: (1) the YSOpopulations in the clouds can be differentiated by the fraction of theirlow-Tbol sources, which increases systematically from Lup and Cha to Tauand to Oph and CrA. This trend is interpreted as increasing currentstar-forming activity in the same order; (2) the clouds with higher coldsource fractions also seem to have higher bright source fractions; (3)In the BLT diagram, the CrA and Oph sources are more uniformlydistributed while the Cha and Lup sources are aggregated near thezero-age mainsequence (ZAMS). Tau sources appear to be an intermediatecase. Taurus also seems to contain more cold (Tbol < 1000 K) andlow-luminosity (Lbol < 1 Lȯ) sources than the other complexes;(4) the YSOs show a characteristic distribution in the median BLTdiagram. This distribution is qualitatively consistent with the earlyYSO evolution from a protostar to a pre--main-sequence star and providesa unique observational test to star-formation models; (5) for Luppre--main-sequence stars, the ratio of their Tbol to Teff increasesduring their approach to the ZAMS. This increase can be explained by thedisk and envelope dissipation during the pre--main-sequence evolution;(6) the most active star-forming clouds (Oph and CrA) also have densermolecular cores as measured by C18O J = 1--0 line emission, suggestingthat the star formation occurs in the densest parts of the molecularclouds; and (7) we find an anti-correlation between Tbol and C18Oemission for the class 0 and I Tau sources (Tbol < 650 K). This showsthat Tbol measures the intrinsic redness of YSOs, rather than theirdisk-envelope orientation. The disk orientation may have a moreimportant effect on Tbol of the pre--main-sequence stars.
| IRAS observations of young stellar objects in the Corona Australis dark cloud
The young stellar object (YSO) population associated with the dark cloudcomplex in Corona Australis is studied by synthesizing IRAS data withnewly obtained near-IR and mid-IR photometry and previously publishedoptical/IR data. Twenty-four YSOs in the Cr A complex are identified.The observed range of spectral energy distribution shapes and bolometricluminosities are consistent with those observed in other dark clouds.The duration and efficiency of star formation are found to be similar tothe Rho Ophiuchi IR cluster. The low number of YSOs compared to otherdark clouds is understood by a reevaluation of the molecular mass of theR Cr A cloud which shows it to be much less massive than previouslyassumed.
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| קבוצת-כוכבים: | כתר דרומי |
| התרוממות ימנית: | 19h04m44.41s |
| סירוב: | -36°50'40.9" |
| גודל גלוי: | 8.129 |
| מרחק: | 185.185 פארסק |
| תנועה נכונה: | 3.7 |
| תנועה נכונה: | -25.4 |
| B-T magnitude: | 8.317 |
| V-T magnitude: | 8.145 |
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