Metal-rich M-dwarf planet hosts: Metallicities with k-band spectra

Ametal-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∼2700) resolutionK-band spectra of 17Mdwarfswith 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 (2.206μm and 2.209μm) and Ca i triplet (2.261μm, 2.263μm, and 2.265μm) 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. Applyin g 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.