Improved Spectroscopic Parameters for Transiting Planet Hosts

Abstract

We report homogeneous spectroscopic determinations of the effective temperature, metallicity, and projected rotational velocity for the host stars of 56 transiting planets. Our analysis is based primarily on the stellar parameter classification (SPC) technique. We investigate systematic errors by examining subsets of the data with two other methods that have often been used in previous studies (Spectroscopy Made Easy (SME) and MOOG). The SPC and SME results, both based on comparisons between synthetic spectra and actual spectra, show strong correlations between T eff , [Fe/H], and log g when solving for all three quantities simultaneously. In contrast the MOOG results, based on a more traditional curve-of-growth approach, show no such correlations. To combat the correlations and improve the accuracy of the temperatures and metallicities, we repeat the SPC analysis with a constraint on log g based on the mean stellar density that can be derived from the analysis of the transit light curves. Previous studies that have not taken advantage of this constraint have been subject to systematic errors in the stellar masses and radii of up to 20% and 10%, respectively, which can be larger than other observational uncertainties, and which also cause systematic errors in the planetary mass and radius.

On the mass estimation for FGK stars: comparison of several methods

“…Unlike other works, the computations based on specific line list of Sousa et al (2008) show little dependence between surface gravities and effective temperature (Mortier et al 2013b). A similar behaviour was also shown in the paper from Torres et al (2012) for a similar procedure also based on equivalent widths. The reason for this is that the surface gravity is constrained using the Fe I lines that are much less in number than the Fe I lines that are used to constrain the effective temperature.…”

SWEET-Cat: A catalogue of parameters for Stars With ExoplanETs

“…Several methodologies have been explored to derive effective temperatures, surface gravities, or stellar metallicities, with clear differences in the resulting zero point or scale (for references and discussions see Sousa et al 2008;Casagrande et al 2010;Torres et al 2012). In other words, it is at present very difficult to point towards any accurate sources of stellar parameters.…”

“…), for this step, we used the PARSEC isochrones (Bressan et al 2012) and a χ 2 minimization process for the fitting. The temperatures and metallicities derived using the ionization and excitation equilibrium of iron lines have been shown to be mostly independent of the adopted surface gravity (Torres et al 2012). This is due to the relatively low sensitivity of Fe i lines (used to constrain the temperature and metallicity) to changes in log g. For example, if we derive the effective temperature and metallicity for the Sun using the adopted methodology and line-lists, but fix log g to 3.0 (a strong ∼1.5 dex Fig.…”

“…Torres et al 2012). The determination of stellar radii depends in turn on the quality of the derived stellar parameters such as the effective temperature.…”

“…In many cases, comparisons have shown that the differences are residual (see e.g. Sousa et al 2008), but in other cases the discrepancies have significant impact on the knowledge of the planet parameters (for a recent discussion on the possible offsets see Torres et al 2012).…”