Source of Atomic Hydrogen in the Atmosphere of HD 209458b

Atomic hydrogen loss at the top of HD 209458b's atmosphere has been recently suggested (Vidal-Madjar et al.). We have developed a one-dimensional model to study the chemistry in the upper atmosphere of this extrasolar "hot Jupiter." The three most abundant elements (other than He) as well as four parent molecules are included in this model, viz., H, C, O, H_2, CO, H_2O, and CH_4. The higher temperatures (~1000 K) and higher stellar irradiance (~6 × 10^5 W m^-2) strongly enhance and modify the chemical reaction rates in this atmosphere. The main result is that the production of atomic hydrogen in the atmosphere is mainly driven by H_2O photolysis, and the reaction of OH with H_2, and is insensitive to the exact abundances of CO, H_2O, and CH_4. For comparison, the bulk H concentration for "hot Jupiters" is 3 orders of magnitude higher than that of Jupiter.