2003ApJ...596L.247L

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<sub>2</sub>, CO, H<sub>2</sub>O, and CH<sub>4</sub>. The higher temperatures (∼1000 K) and higher stellar irradiance (∼6x10<sup>5</sup> W/m2) 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<sub>2</sub>O photolysis, and the reaction of OH with H<sub>2</sub>, and is insensitive to the exact abundances of CO, H<sub>2</sub>O, and CH<sub>4</sub>. For comparison, the bulk H concentration for ``hot Jupiters'' is 3 orders of magnitude higher than that of Jupiter.