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-circle dot and R-* = 1.000(-0.033) (+0.036) R-circle dot, and an evolutionary age of 5.1(-2.3)(+2.7) 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 M-p = 1.198 +/- 0: 053 M-J and Rp = 1.220(- 0.042)(+0.045) R-J, confirming that TrES-2 is the most massive among the currently known nearby (d less than or similar to 300 pc) transiting hot Jupiters. The surface gravity of the planet, log g(p) = 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.