Nearby M subdwarfs from LAMOST data release 2

We identify 108 M subdwarfs(sd Ms) out of more than two hundred thousand M type spectra from the second data release(DR2) of the LAMOSTregular survey. This sample, among which 58 members are identified for the first time, includes 33 extreme subdwarfs(esd Ms) and 11 ultra subdwarfs(usd Ms).The selection is based on the usual ratio of absorption depth of Ca H2, Ca H3 and TiO 5 band systems.We also emphasize the use of the Ca H1 band. We provide estimates of spectral subtype(SPT), L′epine metallicity index ζ, effective temperatureand [Fe/H]. Both ζ–[Fe/H] and SPT–Teff figures show reasonable consistency; compared to PHOENIX model spectra, average rounded values of [Fe/H] for sd Ms, esd Ms and usd Ms are respectively –0.5, –1 and –1.5. The photometric distances are estimated, indicating that most sources are located within 500 pc of the Sun and 350 pc of the Galactic disk. Velocities and 3D Galactic motions are also briefly discussed. Among the 108 subdwarfs, seven stars appear to be active with a significant Hα emission line. The source LAMOSTJ104521.52+482823.3 is a white dwarf- M subdwarfbinary, while LAMOSTJ123045.52+410943.8, also active, exhibits carbon features in red.