Improved stellar parameters of CoRoT-7 A star hosting two super Earths

International audienceContext. Accurate parameters of the host stars of exoplanets are needed for interpreting the new planetary systems. The CoRoT satellite recently discovered a transiting rocky planet with a density similar to the inner planets in our solar system, a so-called super Earth. The mass was determined using ground-based follow-up spectroscopy, which also revealed a second, non-transiting super Earth. Aims. These planets are orbiting a relatively faint (m(V) = 11.7) G9V star called CoRoT-7. We wish to refine the determination of the physical properties of the host star, which are important for the interpretation of the properties of the planetary system. Methods. We used high-quality spectra from HARPS at the ESO3.6 m and UVES at the VLT 8.2 m. We used various methods to analyse the spectra using 1D LTE atmospheric models. From the analysis of Fe I and Fe II lines we determined the effective temperature, surface gravity and microturbulence. We used the Balmer lines to constrain the effective temperature and pressure-sensitive Mg 1b and Ca lines to constrain the surface gravity. We analysed both single spectra and co-add spectra to identify systematic errors. We determine the projected rotational velocity and macroturbulence by fitting the line shapes of isolated lines. We finally determined an approximate absolute magnitude from the Wilson-Bappu effect. Results. From the analysis of the three best spectra, we find T(eff) = 5250 +/- 60 K, log g = 4.47 +/- 0.05, [M/H] = +0.12 +/- 0.06, and v sin i = 1.1(0.5)(+1.0) km s(-1). The chemical composition of 20 analysed elements is consistent with uniform scaling by the metallicity +0.12 dex. From the analysis of spectra of stars with well-known parameters with similar parameters to CoRoT-7 (the Sun and alpha Cen B) we demonstrate that our methods are robust within the claimed uncertainties. We compared the L/M ratio with isochrones to constrain the evolutionary status. Using the age estimate of 1.2-2.3 Gyr based on stellar activity, we determine the mass and radius 0.91 +/- 0.03 M(circle dot) and 0.82 +/- 0.04 R(circle plus). With these updated constraints we fitted the CoRoT transit light curve for CoRoT-7b. The revised planet radius is slightly smaller, R = 1.58 +/- 0.10 R(circle plus), and the density becomes slightly higher, rho = 7.2 +/- 1.8 g cm(-3)