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