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 Micron All Sky Survey (2MASS) J magnitudes are presented for a large sample of 322 nearby candidates from Luyten's NLTT catalogue. Mainly relatively bright ( typically 7 < K-s < 11) and red high proper motion stars have been selected according to their 2MASS magnitudes and optical-to-infrared colours (+ 1 < R- K-s < + 7). Some LHS stars previously lacking spectroscopy have also been included. We have classified the majority of the objects as early-M dwarfs ( M 2-M5). More than 70% of our targets turned out to lie within the 25 pc horizon of the catalogue of nearby stars, with 50 objects placed within 15 pc and 8 objects being closer than 10 pc. Three objects in the 10 pc sample have no previously published spectral type: LP 876-10 (M 4), LP 870-65 (M 4.5), and LP 869-26 (M 5). A large fraction of the objects in our sample (57%) have independent distance estimates, mainly by the recent efforts of Reid and collaborators. Our distance determinations are generally in good agreement with theirs. 11 rather distant (d > 100 pc) objects have also been identified, including a probable halo, but relatively hot (T-eff similar to 13 000 K) white dwarf (LHS 1200) and 10 red dwarfs with extremely large tangential velocities (250 < v(t) < 1150 km s(-1)). Altogether, there are 11 red dwarfs ( including one within 70 pc) with tangential velocities larger than about 250 km s(-1). All these objects are suspected to be in fact subdwarfs, if so, their distances would be only about half of our original estimates. The three most extreme objects in that respect are the K and early M dwarfs LP 323-168, LHS 5343 and LP 552-21 with corrected distances between 180 pc and 400 pc and resulting tangential velocities still larger than about 400 km s(-1).