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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.
| Photometric and spectroscopic studies of cool stars discovered in EXOSAT X-ray images. I - Time variability and spectral classification of eight southern stars
As part of a larger program to study the optical properties ofserendipitous Exosat sources, optical photometry and low-resolutionspectroscopy are presented for eight southern stars which have beenidentified as optical counterparts of Exosat sources. X-ray fluxvariability has been detected for three of them. In all three cases theoptical counterparts show RS CVn-type variability. Of the remaining fivestars, two are found to be variable in the optical. The high X-rayluminosities inferred from the derived spectroscopic parallaxesindicates that these variable sources are all very active systems,possibly RS CVn binaries. The three nonvariable sources are more likelynormal main-sequences stars, two of them with a rather high level ofcoronal emission.
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