Abstract

A metal-rich environment facilitates planet formation, making metal-rich stars the most favorable targets for surveys seeking to detect new exoplanets. Using this advantage to identify likely low-mass planet hosts, however, has been difficult: until now methods to determine M-dwarf metallicities required observationally expensive data (such as parallaxes and high-resolution spectra) and were limited to a few bright cool stars. We have obtained moderate (R similar to 2700) resolution K-band spectra of 17M dwarfs with metallicity estimates derived from their FGK companions. Analysis of these spectra, and inspection of theoretical synthetic spectra, reveals that an M dwarf's metallicity can be inferred from the strength of its Na I doublet (2.206 mu m and 2.209 mu m) and Ca I triplet (2.261 mu m, 2.263 mu m, and 2.265 mu m) absorption lines. We use these features, and a temperature-sensitive water index, to construct an empirical metallicity indicator applicable for M dwarfs with near-solar metallicities (-0.5 [Fe/H] +0.5). This indicator has an accuracy of +/- 0.15 dex, comparable to that of existing techniques for estimating M-dwarf metallicities, but is more observationally accessible, requiring only a moderate resolution K-band spectrum. Applying this method to eight known M-dwarf planet hosts, we estimate metallicities ([Fe/H]) in excess of the mean metallicity of M dwarfs in the solar neighborhood, consistent with the metallicity distribution of FGK planet hosts.