2007ApJ...669.1235L

The current classification system of M stars on the main sequence distinguishes three metallicity classes (dwarfs: dM; subdwarfs: sdM; and extreme subdwarfs: esdM). The spectroscopic definition of these classes is based on the relative strength of prominent CaH and TiO molecular absorption bands near 7000 Å, as quantified by three spectroscopic indices (CaH2, CaH3, and TiO5). The boundaries between the metallicity classes were initially defined from a relatively small sample of only 79 metal-poor stars (subdwarfs and extreme subdwarfs). We re-examine this classification system in light of our ongoing spectroscopic survey of stars with proper motion µ>0.45"/yr, which has increased the census of spectroscopically identified metal-poor M stars to over 400 objects. Kinematic separation of disk dwarfs and halo subdwarfs suggest deficiencies in the current classification system. Observations of common proper motion doubles indicates that the current dM/sdM and sdM/esdM boundaries in the [TiO5, CaH2+CaH3] index plane do not follow isometallicity contours, leaving some binaries inappropriately classified as dM+sdM or sdM+esdM. We propose a revision of the classification system based on an empirical calibration of the TiO/CaH ratio for stars of near solar metallicity. We introduce the parameter ζ_TiO/CaH, which quantifies the weakening of the TiO band strength due to metallicity effect, with values ranging from ζ_TiO/CaH=1 for stars of near-solar metallicity to ζ_TiO/CaH≃0 for the most metal-poor (and TiO depleted) subdwarfs. We redefine the metallicity classes based on the value of the parameter ζ_TiO/CaH and refine the scheme by introducing an additional class of ultrasubdwarfs (usdM). We introduce sequences of sdM, esdM, and usdM stars to be used as formal classification standards.