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The current classification system of M stars on the main sequencedistinguishes three metallicity classes (dwarfs - dM, subdwarfs - sdM, andextreme subdwarfs - esdM). The spectroscopic definition of these classes isbased on the relative strength of prominent CaH and TiO molecular absorptionbands near 7000A, as quantified by three spectroscopic indices (CaH2, CaH3, andTiO5). We re-examine this classification system in light of our ongoingspectroscopic survey of stars with proper motion \mu > 0.45 "/yr, which hasincreased the census of spectroscopically identified metal-poor M stars to over400 objects. Kinematic separation of disk dwarfs and halo subdwarfs suggestdeficiencies in the current classification system. Observations of commonproper motion doubles indicates that the current dM/sdM and sdM/esdM boundariesin the [TiO5,CaH2+CaH3] index plane do not follow iso-metallicity contours,leaving some binaries inappropriately classified as dM+sdM or sdM+esdM. Wepropose a revision of the classification system based on an empiricalcalibration of the TiO/CaH ratio for stars of near solar metallicity. Weintroduce the parameter \zeta_{TiO/CaH} which quantifies the weakening of theTiO bandstrength due to metallicity effect, with values ranging from\zeta_{TiO/CaH}=1 for stars of near-solar metallicity to \zeta_{TiO/CaH}~0 forthe most metal-poor (and TiO depleted) subdwarfs. We redefine the metallicityclasses based on the value of the parameter \zeta_{TiO/CaH}; and refine thescheme by introducing an additional class of ultra subdwarfs (usdM). Weintroduce sequences of sdM, esdM, and usdM stars to be used as formalclassification standards.

Revised metallicity classes for low-mass stars: dwarfs (dM), subdwarfs (sdM), extreme subdwarfs (esdM), and ultra subdwarfs (usdM)