Near-Infrared Spectroscopy of Low-Mass Kepler Planet-Candidate Host Stars: Effective Temperatures, Metallicities, Masses and Radii

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  • 8 September 2011
Abstract
We report stellar parameters for low-mass planet-candidate host stars recently announced by the Kepler Mission. We obtained medium-resolution, K-band spectra of 84 low-mass Kepler Objects of Interest (KOIs). We identified one KOI as a giant; for the remaining dwarfs, we estimated effective temperatures by comparing measurements of K-band regions dominated by H2O opacity with predictions of synthetic spectra for low-mass stars. We measured overall metallicities ([M/H]) using the equivalent widths of Na I and Ca I absorption features and an empirical metallicity relation calibrated with nearby stars. With effective temperatures and metallicities, we estimate the masses and radii of the low-mass KOIs by interpolation onto evolutionary isochrones. The resultant stellar radii are roughly half of the values reported in the Kepler Input Catalogue and, by construction, correlate better with effective temperature. Our results significantly reduce the sizes of the corresponding planet-candidates, with many less than 1 Earth radius. Recalculating the equilibrium temperatures of the planet-candidates from the implied stellar luminosities and masses, and assuming Earth's albedo and re-radiation fraction, we find that six of the planet-candidates are terrestrial-sized with orbital semi-major axes that lie within the habitable zones of their low-mass host stars. The stellar parameters presented in this letter serve as a resource for further characterization of the planet-candidates.

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