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HIGH-RESOLUTION SPECTROSCOPY

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L167The Astrophysical Journal, 616:L167–L170, 2004 December 1 䉷 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.HIGH-RESOLUTION SPECTROSCOPY OF THE TRANSITING PLANET HOST STAR TrES-1Alessandro Sozzetti,1,2David Yong,3Guillermo Torres,2David Charbonneau,2David W. Latham,2Carlos Allende Prieto,4Timothy M. Brown,5Bruce W. Carney,3and John B. Laird6Received 2004 September 25; accepted 2004 October 19; published 2004 October 28ABSTRACTWe report on a spectroscopic determination of the stellar parameters and chemical abundances for the parent starof the transiting planet TrES-1. Based on a detailed analysis of iron lines in our Keck and Hobby-Eberly Telescopespectra, we derive K, , and . By measuring the Ca iiT p 5250 Ⳳ 75 log g p 4.6 Ⳳ 0.2 [Fe/H] p 0.00 Ⳳ 0.09effactivity indicator and by putting useful upper limits on the Li abundance, we constrain the age of TrES-1 to beGyr. By comparing theoretical stellar evolution models with the observational parameters, we obtain2.5 Ⳳ 1.5and . Our improved estimates of the stellar parameters are utilizedM p 0.89 Ⳳ 0.05 MRp 0.83 Ⳳ 0.05 R ,  ,in a new analysis of the transit photometry of TrES-1 to derive a mass , a radiusM p (0.76 Ⳳ 0.05) MRpp J p, and an inclination deg. The improved planetary mass and radius estimates provide the⫹0.08 ⫹0.51.04 Rip 89.5⫺0.05 J ⫺1.3grounds for new crucial tests of theoretical models of evolution and evaporation of irradiated extrasolar giant planets.Subject headings: planetary systems: formation — stars: abundances — stars: individual (TrES-1)Online material: color figure1. INTRODUCTIONThe discovery of an extrasolar Jupiter-sized planet transitingthe disk of the K0 V star TrES-1 (Alonso et al. 2004) hasmarked the first success of ground-based photometric surveystargeting large areas of the sky with small-size telescopessearching for low-amplitude periodic variations in the lightcurves of bright stars ( ). In this Letter we report on aV ≤ 13detailed spectroscopic determination of the stellar parametersand chemical abundances of iron and lithium of the transitingplanet host star.TrES-1 is only the second transiting planet orbiting a starbright enough to allow for a variety of follow-up analyses similarto those conducted for HD 209458b (see, e.g., Charbonneau 2003and references therein). In particular, high-resolution, high signal-to-noise ratio spectroscopic observations of TrES-1 can be read-ily conducted, allowing for improved values of the effectivetemperature, surface gravity, and metallicity of the star. Theseparameters, when compared with stellar evolution models, pro-vide the means to derive good estimates of its mass and radius.Finally, by combining spectroscopic observations with transitphotometry, it is possible to determine refined estimates of themass and radius of the planet. A better knowledge of theseparameters offers us the tantalizing possibility for making crucialtests of giant planet formation, migration, and evolution. In ad-dition, the hypothesis of self-enrichment due to the recent in-gestion of planetary material can be tested using detailed abun-dances of elements such as lithium.1Department of Physics and Astronomy, University of Pittsburgh, 100 AllenHall, 3941 O’Hara Street, Pittsburgh, PA 15260.2Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,Cambridge, MA 02138; [email protected], [email protected],[email protected], [email protected] of Physics and Astronomy, University of North Carolina, CB3255 Phillips Hall, Chapel Hill, NC 27599-3255; [email protected],[email protected] Observatory and Department of Astronomy, University ofTexas, Austin, TX 78712; [email protected] Altitude Observatory/National Center for Atmospheric Research,3450 Mitchell Lane, Boulder, CO 80307; [email protected] of Physics and Astronomy, Bowling Green State University,104 Overman Hall, Bowling Green, OH 43403; [email protected] DATA REDUCTION AND ABUNDANCE ANALYSISThe spectroscopic observations that led to the radial velocityconfirmation of the planetary nature of the transiting object de-tected by the TrES (Trans-atlantic Exoplanet Survey) consortium(Alonso et al. 2004) were performed with the High ResolutionEchelle Spectrometer (HIRES) and its I2absorption cell on theKeck I telescope (Vogt et al. 1994). Eight star⫹iodine spectraand one template spectrum were collected over 18 days in 2004July. The template spectrum used here has a resolution R ⯝and a signal-to-noise ratio pixel⫺1. Three65,000 S/N ⯝ 80spectra were taken with the High Resolution Spec-R ⯝ 60,000trograph (HRS; Tull 1998) on the Hobby-Eberly Telescope(HET) during consecutive nights in 2004 August, with spectralcoverage in the range 5879–7838 . The averaged spectrum has˚Apixel⫺1. For all spectra, thorium-argon lamp expo-S/N ⯝ 120sures provided the wavelength calibration.Our abundance analysis of the Keck/HIRES and HET/HRSspectra of TrES-1 was carried out under the assumption ofstandard local thermodynamic equilibrium (LTE) using amodified version of the spectral synthesis code MOOG (Sneden1973) and a grid of Kurucz (1993) stellar atmospheres. Weselected a set of 30 Fe i and four Fe ii lines (with lowerexcitation potentials eV and0.86 ≤ x ≤ 5.03 2.58 ≤ x ≤lleV, respectively) from the HIRES spectrum and used stan-3.90dard packages in IRAF to derive equivalent widths (EWs) forall of them. Our selection of lines and transition probabilitiesfollowed that of Lee & Carney (2002).3. STELLAR PARAMETERSAlonso et al. (2004) report on the analysis of seven spectraof TrES-1 taken with the CfA Digital Speedometers (Latham1992). By comparing them with a library of synthetic spectra,they derive estimates of the effective temperature, surface grav-ity, and metallicity as follows: K,T p 5250 Ⳳ 200 log g peff, and . As the authors later point out in4.5 Ⳳ 0.5 [Fe/H] ∼ 0.0their discovery paper, these relatively large uncertainties have asignificant impact on the final estimates of the mass and radiusof the transiting planet, and ultimately on the possibility to con-front theory with observation. We describe below the fourfoldstrategy we have devised to better constrain the stellar and con-L168 SOZZETTI ET AL. Vol. 616Fig. 1.—Comparison between the Ca ii H line for TrES-1 (solid line) andan inactive star of the same temperature (dotted line).sequently planetary


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