Calibrating M-Dwarf Metallicities Using Molecular Indices: Extension to Low-metallicity Stars

We have calculated Fe and Ti abundances in 12 low-metallicity main sequence M stars using high-resolution spectra. These subdwarf and extreme-subdwarf stars allow us to extend our calibration of a method to determine cool-dwarf-star metallicities using molecular band strength indices from low-resolution spectra. Our calibration can now be used to determine metallicity to within +/- 0.3 dex for stars with [Fe/H] between -1.5 and +0.05 and temperatures between 3500 and 4000 K. We also report a method to estimate temperatures for M dwarfs using equivalent width measurements of the infrared Ca II triplet and the K I line at 7699 angstrom. Our metallicity measurements show that the recently proposed classification system for low-mass stars (dwarfs, subdwarfs, extreme subdwarfs, and ultrasubdwarfs) does represent a metallicity sequence, with the ultrasubdwarfs the most metal-poor stars.