Characterizing the Thermosphere of HD209458b with UV Transit Observations

Transmission spectroscopy at UV wavelengths is a rich and largely unexplored source of information about the upper atmospheres of extrasolar planets. So far, UV transit observations have led to the detection of atomic hydrogen, oxygen, and ionized carbon in the upper atmosphere of HD209458b. The interpretation of these observations is controversial--it is not clear if the absorption arises from an escaping atmosphere interacting with the stellar radiation and stellar wind, or from the thermosphere inside the Roche lobe. In this paper, we introduce an empirical model that can be used to analyze UV transit depths of extrasolar planets. We use this model to interpret the transits of HD209458b in the H I 1216 and the O I 1304 triplet emission lines. The results indicate that the mean temperature of the thermosphere is T = 8000-11,000 K and that the H2/H dissociation front is located at pressures between p = 0.1 and 1 Delta *mbar, which correspond to a distance r 1.1 Rp from the center of the planet. The upper boundary of the model thermosphere is located at r = 2.7-3 Rp , above which the atmosphere is mostly ionized. We find that the H I transit depth in the wings of the H Ly Delta *a line reflects the optical depth of the thermosphere, but that the atmosphere also overflows the Roche lobe. By assuming a solar mixing ratio of oxygen, we obtain an O I transit depth that is statistically consistent with the observations. An O I transit depth comparable to the H I transit depth is possible if the atmosphere is undergoing fast hydrodynamic escape, the O/H ratio is supersolar, or if a significant quantity of neutral oxygen is found outside the Roche lobe. We find that the observations can be explained solely by absorption in the upper atmosphere and extended clouds of energetic neutral atoms or atoms strongly perturbed by radiation pressure are not required. Due to the large uncertainty in the data, repeated observations are necessary to better constrain the O I transit depths and thus the composition of the thermosphere.