SPECTROSCOPY OF FAINT KEPLER MISSION EXOPLANET CANDIDATE HOST STARS

Stellar properties are measured for a large set of Kepler mission exoplanet candidate host stars. Most of these stars are fainter than 14th magnitude, in contrast to other spectroscopic follow-up studies. This sample includes many high-priority Earth-sized candidate planets. A set of model spectra are fitted to R similar to 3000 optical spectra of 268 stars to improve estimates of T-eff, log( g), and [Fe/H] for the dwarfs in the range 4750 <= T-eff <= 7200 K. These stellar properties are used to find new stellar radii and, in turn, new radius estimates for the candidate planets. The result of improved stellar characteristics is a more accurate representation of this Kepler exoplanet sample and identification of promising candidates for more detailed study. This stellar sample, particularly among stars with T-eff greater than or similar to 5200 K, includes a greater number of relatively evolved stars with larger radii than assumed by the mission on the basis of multi-color broadband photometry. About 26% of the modeled stars require radii to be revised upward by a factor of 1.35 or greater, and modeling of 87% of the stars suggest some increase in radius. The sample presented here also exhibits a change in the incidence of planets larger than 3-4 R-circle plus as a function of metallicity. Once [Fe/H] increases to >= -0.05, large planets suddenly appear in the sample while smaller planets are found orbiting stars with a wider range of metallicity. The modeled stellar spectra, as well as an additional 84 stars of mostly lower effective temperatures, are made available to the community.