Improved stellar parameters of CoRoT-7^*

A star hosting two super Earths

¹ LESIA, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon, France e-mail: bruntt@phys.au.dk
² LAM, UMR 6110, CNRS/Université de Provence, 38 rue F. Joliot-Curie, 13388 Marseille, France
³ ESA, ESTEC, SRE-SA, Keplerlaan 1, 2200AG, Noordwijk, The Netherlands
⁴ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁵ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis Bd Arago, 75014 Paris, France
⁶ Observatoire de Haute-Provence, CNRS/OAMP, 04870 St Michel l'Observatoire, France
⁷ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany

Received: 27 January 2010
Accepted: 17 May 2010

Abstract

Context. 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 ESO 3.6 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 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 α 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  and 0.82±0.04 . 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 , and the density becomes slightly higher, ρ = 7.2±1.8  g cm^-3.