Spectroscopy of Faint Kepler Mission Exoplanet Candidate Host Stars

TL;DR

This study provides improved stellar parameters for faint Kepler exoplanet host stars, refining planet size estimates and revealing correlations with stellar metallicity, thereby enhancing the characterization of the exoplanet sample.

Contribution

It introduces a spectroscopic analysis of faint Kepler stars, improving stellar and planetary radius estimates and exploring planet-metallicity relationships.

Findings

26% of stars require larger radii by a factor of 1.35 or more.

87% of stars show some increase in estimated radius.

Large planets (>3-4 Earth radii) are more common around stars with higher metallicity.

Abstract

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~3000 optical spectra of 268 stars to improve estimates of Teff, log(g), and [Fe/H] for the dwarfs in the range 4750K<Teff<7200K. 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 Teff>5200K, 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 modelled stars require radii to be revised upwards by a factor of 1.35 or greater, and modelling 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 Earth radii 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 modelled stellar spectra, as well as an additional 84 stars of mostly lower effective temperatures, are made available to the community.