Observation of the full 12-hour-long transit of the exoplanet HD 80606b^*

Warm-Spitzer photometry and SOPHIE spectroscopy

¹ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France e-mail: hebrard@iap.fr
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Observatoire de Haute-Provence, CNRS/OAMP, 04870 Saint-Michel-l'Observatoire, France
⁴ Laboratoire d'Astrophysique de Marseille, Univ. de Provence, CNRS (UMR6110), 38 rue F. Joliot Curie, 13388 Marseille, France
⁵ Laboratoire d'Astrophysique de Grenoble, Université Joseph-Fourier, CNRS (UMR5571), BP 53, 38041 Grenoble Cedex 9, France
⁶ Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
⁷ CROW-observatory Portalegre, Portugal, and Atalaia.org group, Portugal
⁸ School of Physics, University of Exeter, Exeter, EX4 4QL, UK
⁹ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal

Received: 26 February 2010
Accepted: 5 April 2010

Abstract

We present new observations of a transit of the 111.4-day-period exoplanet HD 80606b. Due to this long orbital period and to the orientation of the eccentric orbit (e = 0.9), HD 80606b's transits last for about 12 hours. This makes the observation of a full transit practically impossible from a given ground-based observatory. With the Spitzer Space Telescope and its IRAC camera on the post-cryogenic mission, we performed a 19-h photometric observation of HD 80606 that covers the full 2010 January 13-14 transit as well as off-transit references immediately before and after the event. We complement these photometric data by new spectroscopic observations that we simultaneously performed with SOPHIE at the Haute-Provence Observatory. This provides radial velocity measurements of the first half of the transit that was previously uncovered with spectroscopy. This new dataset allows the parameters of this singular planetary system to be significantly refined. We obtained a planet-to-star radius ratio R_p/R_* = 0.1001 ± 0.0006 that is more accurate but slightly lower than the one measured from previous ground observations in the optical. We found no astrophysical interpretations able to explain this difference between optical and infrared radii; we rather favor underestimated systematic uncertainties, maybe in the ground-based composite light curve. We detected a feature in the Spitzer light curve that could be due to a stellar spot. We also found a transit timing about 20 minutes earlier than the ephemeris prediction; this could be caused by actual transit-timing variations due to an additional body in the system, or again by underestimated systematic uncertainties. The actual angle between the spin-axis of HD 80606 and the normal to the planetary orbital plane is found to be near 40° thanks to the fit of the Rossiter-McLaughlin anomaly, with a sky-projected value λ = 42° ± 8°. This allows scenarios with aligned spin-orbit to be definitively rejected. Over the twenty planetary systems with measured spin-orbit angles, a few are misaligned; this is probably the signature of two different evolution scenarios for misaligned and aligned systems, depending whether or not they experienced gravitational interaction with a third body. As in the case of HD 80606, most of the planetary systems including a massive planet are tilted; this could be the signature of a separate evolution scenario for massive planets compared with Jupiter-mass planets.