Volume 514, May 2010
|Number of page(s)||10|
|Section||Planets and planetary systems|
|Published online||21 May 2010|
Evaporation of the planet HD 189733b observed in H I Lyman-α
CNRS, UMR 7095,
Institut d'astrophysique de Paris,
98bis boulevard Arago, 75014 Paris, France e-mail: firstname.lastname@example.org
2 UPMC Univ. Paris 6, UMR 7095, Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
3 Université Joseph Fourier-Grenoble 1/CNRS, Laboratoire d'astrophysique de Grenoble (LAOG) UMR 5571, BP 53, 38041 Grenoble Cedex 09, France
4 Lunar and Planetary Laboratory, University of Arizona, 1541 E. University Blvd., Tucson, AZ 85721-0063, USA
5 Astrophysics Group, School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
6 Lycée Passy Buzenval, 50 avenue Otis Mygatt, 92508 Rueil-Malmaison Cedex, France
7 Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
Accepted: 8 February 2010
We observed three transits of the extrasolar planet HD 189733b in H i Lyman-α and in a few other lines in the ultraviolet with HST/ACS, in the search for atmospheric signatures. We detect a transit signature in the Lyman-α light curve with a transit depth of 5.05 ± 0.75%. This depth exceeds the occultation depth produced by the planetary disk alone at the 3.5σ level (statistical). Other stellar emission lines are less bright, and, taken individually, they do not show the transit signature, while the whole spectra redward of the Lyman-α line has enough photons to show a transit signature consistent with the absorption by the planetary disk alone. The transit depth's upper limits in the emission lines are 11.1% for O i λ1305 Å and 5.5% for C ii λ1335 Å lines. The presence of an extended exosphere of atomic hydrogen around HD 189733b producing 5% absorption of the full unresolved Lyman-α line flux shows that the planet is losing gas. The Lyman-α light curve is well-fitted by a numerical simulation of escaping hydrogen in which the planetary atoms are pushed by the stellar radiation pressure. We constrain the escape rate of atomic hydrogen to be between 109 and 1011 g s-1 and the ionizing extreme UV flux between 2 and 40 times the solar value (1-σ), with higher escape rates corresponding to larger EUV flux. The best fit is obtained for dM/dt = 1010 g s-1 and an EUV flux FEUV = 20 times the solar value. HD 189733b is the second extrasolar planet for which atmospheric evaporation has been detected.
Key words: planetary systems / stars: individual: HD 189733
© ESO, 2010
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