Protostars and Planets VI, Heidelberg, July 15-20, 2013
H-alpha ABSORPTION IN TRANSITING EXOPLANET ATMOSPHERES
Christie, Duncan (University of Virginia)
Arras, Phil (University of Virginia)
Li, Zhi-Yun (University of Virginia)
While Lyman-alpha absorption provides a probe of the hydrogen content of an exoplanet atmosphere, the recent observation of H-alpha absorption by the atmosphere of HD189733b provides an opportunity to probe the thermal structure of the atomic hydrogen as well. With this in mind, we construct a spherically-symmetric atmosphere in hydrostatic, thermal, and photoionization equilibrium. Including collisional excitation, photoionization, radiative recombination, as well as radiative transitions, we calculate the spatial distribution of n=2 hydrogen within the model atmosphere. We find that the H-alpha absorption signal arises from metastable 2s hydrogen within the neutral layer between micro-bar and nano-bar pressures. Within this layer, the number density of 2s varies slowly, despite the 1s number density falling by 2-3 orders of magnitude. With this model, we demonstrate that the H-alpha integrated transit depths for HD189733b can be explained using a simple model of thermal gas in the extended planetary atmosphere.
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