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

► We developed a new model to explain the FUV transit observations of HD209458b. ► We find a qualitative agreement between the model and the observed atmosphere. ► We constrain the mass loss rate and escape mechanism based on stellar heating. ► We present density profiles for the detected heavy atoms and ions. ► Diffusive separation of heavy species is prevented by the escape of H and protons.