WASP-71b: a bloated hot Jupiter in a 2.9-day, prograde orbit around an evolved F8 star^⋆

¹ Astrophysics Group, Lennard-Jones LaboratoriesKeele University, Keele, Staffordshire ST5 5BG, UK
² N. Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
e-mail: amss@camk.edu.pl
³ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
⁴ Observatoire de Haute-Provence, CNRS/OAMP, 04870 Saint-Michel-l’Observatoire, France
⁵ SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS, UK
⁶ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Sart Tilman, Liège 1, Belgium
⁷ Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
⁸ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹⁰ Department of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK

Received: 12 November 2012
Accepted: 14 March 2013

Abstract

We report the discovery by the WASP transit survey of a highly-irradiated, massive (2.242 ± 0.080 M_Jup) planet which transits a bright (V = 10.6), evolved F8 star every 2.9 days. The planet, WASP-71b, is larger than Jupiter (1.46 ± 0.13 R_Jup), but less dense (0.71 ± 0.16 ρ_Jup). We also report spectroscopic observations made during transit with the CORALIE spectrograph, which allow us to make a highly-significant detection of the Rossiter-McLaughlin effect. We determine the sky-projected angle between the stellar-spin and planetary-orbit axes to be λ = 20.1 ± 9.7 degrees, i.e. the system is “aligned”, according to the widely-used alignment criteria that systems are regarded as misaligned only when λ is measured to be greater than 10 degrees with 3-σ confidence. WASP-71, with an effective temperature of 6059 ± 98 K, therefore fits the previously observed pattern that only stars hotter than 6250 K are host to planets in misaligned orbits. We emphasise, however, that λ is merely the sky-projected obliquity angle; we are unable to determine whether the stellar-spin and planetary-orbit axes are misaligned along the line-of-sight. With a mass of 1.56 ± 0.07 M_⊙, WASP-71 was previously hotter than 6250 K, and therefore might have been significantly misaligned in the past. If so, the planetary orbit has been realigned, presumably through tidal interactions with the cooling star’s growing convective zone.