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 \AA , 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 \mu > 0.45 \arcsec yr ^ { -1 } , 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 iso-metallicity 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 \zeta _ { TiO / CaH } which quantifies the weakening of the TiO bandstrength due to metallicity effect , with values ranging from \zeta _ { TiO / CaH } = 1 for stars of near-solar metallicity to \zeta _ { TiO / CaH } \simeq 0 for the most metal-poor ( and TiO depleted ) subdwarfs .
We redefine the metallicity classes based on the value of the parameter \zeta _ { TiO / CaH } ; and refine the scheme by introducing an additional class of ultra subdwarfs ( usdM ) .
We introduce sequences of sdM , esdM , and usdM stars to be used as formal classification standards .