Metal-rich M-Dwarf Planet Hosts: Metallicities with K-band Spectra

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 ~ 2700) resolution K-band spectra of 17 M 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 μm and 2.209 μm) and Ca I triplet (2.261 μm, 2.263 μm, and 2.265 μm) 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.