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 _ { \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 . ( 2007b ) and our T _ { \mathrm { eff } } estimate we obtain M _ { \star } = 0.980 \pm 0.062 ~ { } M _ { \odot } and R _ { \star } = 1.000 _ { -0.033 } ^ { +0.036 } ~ { } R _ { \odot } , and an evolutionary age of 5.1 ^ { +2.7 } _ { -2.3 } Gyr , in good agreement with other constraints based on the strength of the emission in the Ca ii 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 0.053 ~ { } M _ { \mathrm { Jup } } and R _ { p } = 1.220 ^ { +0.045 } _ { -0.042 } ~ { } R _ { \mathrm { Jup } } , confirming that TrES-2 is the most massive among the currently known nearby ( d \lesssim 300 pc ) transiting hot Jupiters .
The surface gravity of the planet , \log g _ { p } = 3.299 \pm 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 .