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The detections of atomic hydrogen, heavy atoms and ions surrounding theextrasolar giant planet (EGP) HD209458b constrain the composition, temperatureand density profiles in its upper atmosphere. Thus the observations provideguidance for models that have so far predicted a range of possible conditions.We present the first hydrodynamic escape model for the upper atmosphere thatincludes all of the detected species in order to explain their presence at highaltitudes, and to further constrain the temperature and velocity profiles. Thismodel calculates the stellar heating rates based on recent estimates ofphotoelectron heating efficiencies, and includes the photochemistry of heavyatoms and ions in addition to hydrogen and helium. The composition at the lowerboundary of the escape model is constrained by a full photochemical model ofthe lower atmosphere. We confirm that molecules dissociate near the 1 microbarlevel, and find that complex molecular chemistry does not need to be includedabove this level. We also confirm that diffusive separation of the detectedspecies does not occur because the heavy atoms and ions collide frequently withthe rapidly escaping H and H+. This means that the abundance of the heavy atomsand ions in the thermosphere simply depends on the elemental abundances andionization rates. We show that, as expected, H and O remain mostly neutral upto at least 3 Rp, whereas both C and Si are mostly ionized at significantlylower altitudes. We also explore the temperature and velocity profiles, andfind that the outflow speed and the temperature gradients depend strongly onthe assumed heating efficiencies...

The escape of heavy atoms from the ionosphere of HD209458b. I. A photochemical-dynamical model of the thermosphere