The escape of heavy atoms from the ionosphere of HD209458b. II. Interpretation of the observations

► We compare UV transits of HD209458b with empirical and hydrodynamic models. ► We constrain the mean temperature, densities and escape rates of different species. ► The detection of atomic oxygen implies a minimum mass loss rate of 6 × 106 kg s−1. ► The results constrain the temperature, chemistry, and ionization of the atmosphere. ► Detection of Si2+ indicates that clouds of forsterite and enstatite do not form.