Accurate parameters of 93 solar-type Kepler targets

TL;DR

This study provides precise spectroscopic parameters and chemical compositions for 93 Kepler solar-type stars, validating methods for effective temperature and surface gravity determination, and discusses metallicity and alpha-element trends.

Contribution

It offers the first detailed chemical analysis of a large sample of Kepler targets with asteroseismic surface gravity, confirming the reliability of spectroscopic and photometric temperature scales.

Findings

Fe lines accurately represent overall metallicity

Good agreement between spectroscopic and asteroseismic log g values

Alpha-enhancement is significant only below -0.3 metallicity

Abstract

We present a detailed spectroscopic study of 93 solar-type stars that are targets of the NASA/Kepler mission and provide detailed chemical composition of each target. We find that the overall metallicity is well-represented by Fe lines. Relative abundances of light elements (CNO) and alpha-elements are generally higher for low-metallicity stars. Our spectroscopic analysis benefits from the accurately measured surface gravity from the asteroseismic analysis of the Kepler light curves. The log g parameter is known to better than 0.03 dex and is held fixed in the analysis. We compare our Teff determination with a recent colour calibration of V-K (TYCHO V magnitude minus 2MASS Ks magnitude) and find very good agreement and a scatter of only 80 K, showing that for other nearby Kepler targets this index can be used. The asteroseismic log g values agree very well with the classical determination using Fe1-Fe2 balance, although we find a small systematic offset of 0.08 dex (asteroseismic log g values are lower). The abundance patterns of metals, alpha elements, and the light elements (CNO) show that a simple scaling by [Fe/H] is adequate to represent the metallicity of the stars, except for the stars with metallicity below -0.3, where alpha-enhancement becomes important. However, this is only important for a very small fraction of the Kepler sample. We therefore recommend that a simple scaling with [Fe/H] be employed in the asteroseismic analyses of large ensembles of solar-type stars.