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

Abstract

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 \ion{Fe}{1} and \ion{Fe}{2} lines from our Keck spectra yields $T_{\mathrm{eff}}=5850\pm 50$ K, $\log g=4.4\pm 0.1$, and [Fe/H] $=-0.15\pm 0.10$. Several independent checks (e.g., additional spectroscopy, line-depth ratios) confirm the reliability of our spectroscopic $T_{\mathrm{eff}}$ 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 $T_{\mathrm{eff}}$ and some measure of the stellar luminosity, such as the spectroscopic surface gravity (when a trigonometric parallax is unavailable, as in this case). We apply here a new method in which we use instead of $\log g$ the normalized separation $a/R_{\star }$ (related to the stellar density), which can be determined directly from the light curves of transiting planets with much greater precision. With the $a/R_{\star }$ value from the light curve analysis of Holman et al. \citeyearpar{holman07b} and our $T_{\mathrm{eff}}$ estimate we obtain $M_{\star }=0.980\pm 0.062M_{\odot }$ and $R_{\star }=1.000_{-0.033}^{+0.036}{R}_{\odot }$, and an evolutionary age of $5.1_{-2.3}^{+2.7}$ Gyr, in good agreement with other constraints based on the strength of the emission in the \ion{Ca}{2} H & K line cores, the Lithium abundance, and rotation. The new stellar parameters yield improved values for the planetary mass and radius of $M_p=1.198\pm 0.053M_{\mathrm{Jup}}$ and $R_p=1.220_{-0.042}^{+0.045}{R}_{\mathrm{Jup}}$, confirming that {TrES-2} is the most massive among the currently known nearby ($d\lesssim 300$ pc) transiting hot Jupiters. [Abridged]

27 pages, 5 figures, 1 table. Accepted for publication in the Astrophysical Journal

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