Metallicities of M Dwarf Planet Hosts from Spectral Synthesis

We present the first spectroscopic metallicities of three M dwarfs with knownor candidate planetary mass companions. We have analyzed high resolution, highsignal-to-noise spectra of these stars which we obtained at McDonaldObservatory. Our analysis technique is based on spectral synthesis of atomicand molecular features using recently revised cool-star model atmospheres andspectrum synthesis code. The technique has been shown to yield resultsconsistent with the analyses of solar-type stars and allows measurements of Mdwarf [M/H] values to 0.12 dex precision. From our analysis, we find [M/H] =-0.12, -0.32, and -0.33 for GJ 876, GJ 436, and GJ 581 respectively. Thesethree M dwarf planet hosts have sub-solar metallicities, a surprising departurefrom the trend observed in FGK-type stars. This study is the first part of ourongoing work to determine the metallicities of the M dwarfs included in theMcDonald Observatory planet search program.Comment: 13 pages, 2 figures, accepted for publication in ApJ