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Southern Very Low Mass Stars and Brown Dwarfs in Wide Binary and Multiple Systems
The results of the Königstuhl survey in the Southern Hemisphere arepresented. I have searched for common proper motion companions to 173field very low mass stars and brown dwarfs with spectral types >M5.0V and magnitudes J<~14.5 mag. I have measured for the first time thecommon proper motion of two new wide systems containing very low masscomponents, Königstuhl 2 AB and 3 A-BC. Together withKönigstuhl 1 AB and 2M 0126-50 AB, they are among the widestsystems in their respective classes (r=450-11,900 AU). I have determinedthe minimum frequency of field wide multiples (r>100 AU) withlate-type components at 5.0%+/-1.8% and the frequency of field widelate-type binaries with mass ratios q>0.5 at 1.2%+/-0.9%. Thesevalues represent a key diagnostic of evolution history and low-mass starand brown dwarf formation scenarios. In addition, the proper motions of62 field very low mass dwarfs are measured here for the first time.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Subdwarf studies. II - Abundances and kinematics from medium resolution spectra. III - The halo metallicity distribution
Stars previously identified as having UV excesses are observed at 1-Aresolution in the Ca II K-line region. Comparisons of these data withother samples and with Monte Carlo simulations involving a singlecomponent halo have yielded estimates of halo velocity dispersions androtation velocity, corrected for the kinematic biases in the sample. Itis suggested that the data are not consistent with a model in which thehalo formed from star formation in a dissipating, collapsing cloud; theyare, however, reconcilable with the formation of the halo stars bynumerous, independently evolving gas clouds. The metallicitydistribution of a sample of 372 kinematically selected halo stars isthen constructed, with a view to selection effects in the data. Goodagreement is noted between the globular cluster metallicity distributionand a stochastic model with a mean of 10 enrichments/fragment.
| Subdwarf studies. I - UBVRI photometry of NLTT stars
UBVRI photometry is presented for a sample of 1656 southern stars,including 1211 that were previously unmeasured, drown from the NLTTproper-motion catalog. The catalog is shown to be a rich source ofsubdwarfs. The normalized ultraviolet excess delta (U - B)0.6,photometric parallax, and interstellar reddening are calculated for eachstar when possible. Photometric parallaxes are compared withtrigonometric parallaxes from the literature. It is found that theformer do not have systematic errors greater than about 25 percent. Inagreement with other studies, the bluest subdwarfs are found at B - V =0.35. The selection of the program stars on the basis of large reducedproper motions restricted subgiant contamination of the sample to about5 percent and increased the discovery fraction of halo stars relative todisk stars. The claim is made here that the sample can be used toinvestigate the abundance distribution of the halo. The sample includesstars with ultraviolet excesses characteristic of disk abundances butwith velocities up to 150 km/s. These are believed to be stars that,quite expectedly, reside in the high-velocity tail of the disk velocitydistribution.
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Osservazione e dati astrometrici
| Costellazione: | Corvo |
| Ascensione retta: | 12h36m59.33s |
| Declinazione: | -21°20'38.3" |
| Magnitudine apparente: | 10.818 |
| Moto proprio RA: | -172.3 |
| Moto proprio Dec: | -48 |
| B-T magnitude: | 11.341 |
| V-T magnitude: | 10.862 |
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