Refined parameters and spectroscopic transit of the supermassive planet HAT-P-2 b ⋆

In this paper we report a refined determination of the orbital parameters and the detection of the Rossiter-McLaughlin e ffect of the recently discovered transiting exoplanet HAT-P-2b. Th e large orbital eccentricity and short orbital period of thi s exoplanet are unexpected and distinguish it from other known transiting e xoplanets. We performed high precision radial velocity spe ctroscopic observations of HD147506 (HAT-P-2) with the new SOPHIE spec trograph, mounted on the 1.93 m telescope at OHP. We obtained 63 new measurements, including 35 on May 14 th and 20 on June 11 , at nights that the planet was transiting its star. The RV ano maly observed illustrates that HAT-P-2b orbital motion is set in the same direction as its parent star spin. The sky-projecte d angle between the normal of the orbital plane and the stellar spin axis, λ = 0.0 ± 9.2, is consistent with zero. The planetary and stellar radii we re re-determined: Rp = 0.952+0.020 −0.027 RJup, Rs = 1.412 +0.029 −0.040 R⊙. The refined values of HAT-P-2b radius and mass ( Mp = 8.64 +0.29 −0.44 MJup) indicate a density of 12 .41+0.42 −0.65 g cm −3, suggesting an object in between the known close-in planets with typical density of the order of 1 g cm−3, and the very low-mass stars, with density greater than 50 g c m−3.