Evaporation of the planet HD 189733b observed in H I Lyman-<i>α</i>

Abstract

We observed three transits of the extrasolar planet HD 189733b in H i Lyman-<i>α<i/> 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-<i>α<i/> light curve with a transit depth of 5.05 <i>±<i/> 0.75%. This depth exceeds the occultation depth produced by the planetary disk alone at the 3.5<i>σ<i/> 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-<i>α<i/> 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 <i>λ<i/>1305 Å and 5.5% for C ii <i>λ<i/>1335 Å lines. The presence of an extended exosphere of atomic hydrogen around HD 189733b producing 5% absorption of the full unresolved Lyman-<i>α<i/> line flux shows that the planet is losing gas. The Lyman-<i>α<i/> 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<sup>9<sup/> and 10<sup>11<sup/> g s<sup>-1<sup/> and the ionizing extreme UV flux between 2 and 40 times the solar value (1-<i>σ<i/>), with higher escape rates corresponding to larger EUV flux. The best fit is obtained for d<i>M<i/>/d<i>t<i/> = 10<sup>10<sup/> g s<sup>-1<sup/> and an EUV flux <i>F<i/><sub>EUV<sub/> = 20 times the solar value. HD 189733b is the second extrasolar planet for which atmospheric evaporation has been detected.