Source of Atomic Hydrogen in the Atmosphere of HD 209458b

Atomic hydrogen loss at the top of HD 209458b's atmosphere has been recentlydetected Vidal-Madjar et al. 2003. We have developed a 1-dimensional model tostudy the chemistry in the upper atmosphere of this extrasolar "hot jupiter".The 3 most abundant elements (other than He), as well as 4 parent molecules areincluded in this model, viz., H, C, O, H2, CO, H2O, and CH4. The highertemperatures (~ 1000 K) and higher stellar irradiance (~6x10^5 W m^{-2})strongly enhance and modify the chemical reaction rates in this atmosphere. Ourtwo main results are that (a) the production of atomic hydrogen in theatmosphere is mainly driven by H2O photolysis and reaction of OH with H2, andis not sensitive to the exact abundances of CO, H2O, and CH4, and (b) H2O andCH4 can be produced via the photolysis of CO followed by the reactions with H2.Comment: submitted to ApJ