Improving Stellar and Planetary Parameters of Transiting Planet Systems: The Case of TrES‐2

We report on a spectroscopic determination of the atmospheric parameters and chemical abundance of the parent star of the recently discovered transiting planet TrES-2. A detailed LTE analysis of a set of Fe I and Fe II lines from our Keck spectra yields T_ (eff) = 5850 ± 50 K, log g = 4. 4 ± 0. 1, and Fe/H = -0. 15 ± 0. 10. Several independent checks (e. g. , additional spectroscopy, line-depth ratios) confirm the reliability of our spectroscopic T_ (eff) estimate. The mass and radius of the star, needed to determine the properties of the planet, are traditionally inferred by comparison with stellar evolution models using T_ (eff) and some measure of the stellar luminosity, such as the spectroscopic surface gravity. We apply here a new method in which we use instead of log g the normalized separation a/R_* (related to the stellar density), directly measurabele from the light curves of transiting planets with much greater precision. With the a/R_* value from the light-curve analysis of Holman and coworkers and our T_ (eff) estimate, we obtain M_* = 0. 980 ± 0. 062 M_☉ and R_* = 1. 000^ (+0. 036) _ (-0. 033) R_☉, and an evolutionary age of 5. 1^ (+2. 7) _ (-2. 3) Gyr, in good agreement with other constraints (Ca II H and K line cores, lithium abundance, and rotation). The new stellar parameters yield improved values for the planetary mass and radius of Mₚ = 1. 198 ± 0. 053 MJ and Rₚ = 1. 220^ (+0. 045) _ (-0. 042) RJ, confirming that TrES-2 is the most massive among the currently known nearby (d ≲ 300 pc) transiting hot Jupiters. The surface gravity of the planet, log gₚ = 3. 299 ± 0. 016, can be derived independently of the knowledge of the stellar parameters (i. e. , directly from observations), and with a very high precision rivaling that of the best known double-lined eclipsing binaries.