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Search for nearby stars among proper motion stars selected by optical-to-infrared photometry. III. Spectroscopic distances of 322 NLTT stars
Distance estimates based on low-resolution spectroscopy and Two MicronAll Sky Survey (2MASS) J magnitudes are presented for a large sample of322 nearby candidates from Luyten's NLTT catalogue. Mainly relativelybright (typically 7 < Ks < 11) and red high propermotion stars have been selected according to their 2MASS magnitudes andoptical-to-infrared colours (+1 < R-Ks < +7). Some LHSstars previously lacking spectroscopy have also been included. We haveclassified the majority of the objects as early-M dwarfs (M 2-M 5). Morethan 70% of our targets turned out to lie within the 25 pc horizon ofthe catalogue of nearby stars, with 50 objects placed within 15 pc and 8objects being closer than 10 pc. Three objects in the 10 pc sample haveno previously published spectral type: LP 876-10 (M 4), LP 870-65 (M4.5), and LP 869-26 (M 5). A large fraction of the objects in our sample(57%) have independent distance estimates, mainly by the recent effortsof Reid and collaborators. Our distance determinations are generally ingood agreement with theirs. 11 rather distant (d > 100 pc) objectshave also been identified, including a probable halo, but relatively hot(T_eff13 000 K) white dwarf (LHS 1200) and 10 red dwarfs withextremely large tangential velocities (250 < vt < 1150km s-1). Altogether, there are 11 red dwarfs (including onewithin 70 pc) with tangential velocities larger than about 250 kms-1. All these objects are suspected to be in fact subdwarfs,if so, their distances would be only about half of our originalestimates. The three most extreme objects in that respect are the K andearly M dwarfs LP 323-168, LHS 5343 and LP 552-21 with correcteddistances between 180 pc and 400 pc and resulting tangential velocitiesstill larger than about 400 km s-1.
| 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.
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