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Observation of the full 12-hour-long transit of the exoplanet HD80606b

G. Hébrard, J.-M. Désert, R. F. Díaz, I. Boisse, F. Bouchy, A. Lecavelier Des Etangs, C. Moutou, D. Ehrenreich, L. Arnold, X. Bonfils, X. Delfosse, M. Desort, A. Eggenberger, T. Forveille, J. Gregorio, A.-M. Lagrange, C. Lovis, F. Pepe, C. Perrier, F. Pont, D. Queloz, A. Santerne, N. C. Santos, D. Ségransan, D. K. Sing, S. Udry, A. Vidal-Madjar

Abstract
We present new observations of a transit of the 111.4-day-period exoplanet HD80606b. Due to this long orbital period and to the orientation of the eccentric orbit (e = 0.9), HD80606b'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 postcryogenic mission, we performed a 19-h photometric observation of HD80606 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 Rp/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 HD80606 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 HD80606, 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.

Keywords
techniques: radial velocities - techniques: photometric - stars: individual: HD80606

Notes
Based on observations collected with the SOPHIE spectrograph on the 1.93-m telescope at the Observatoire de Haute-Provence (CNRS), France, and with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Radial velocity and photometry tables are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/516/A95

Astronomy and Astrophysics
Volume 516, Number of pages A95_1
2010 June

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Fundação para a Ciência e a Tecnologia
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