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, logg=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 logg 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_solar and R_*=1.000^+0.036_-0.033 R_solar, 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 M_p=1.198+/-0.053 M_J and R_p=1.220^+0.045_-0.042 R_J, 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, logg_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.