| Abstract
| - 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 3.5 σ 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 iλ1305 Å and 5.5% for C iiλ1335 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 10 9 and 10 11 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 d M/d t = 10 10 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.
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