Suprathermal hydrogen produced by the dissociation of molecular hydrogen in the extended atmosphere of exoplanet HD 209458b

Abstract

The ionization and dissociation of molecular hydrogen by the ultraviolet (UV) radiation of the parent star lead to the formation of hydrogen atoms with an excess of kinetic energy and, thus, are an important source of suprathermal hydrogen atoms in the upper atmosphere of exoplanet HD 209458b. Contemporary aeronomical models did not investigate these processes because they assumed the fast local thermalization of the hot atoms of hydrogen by elastic collisions. However, the kinetics and transfer of these atoms were not calculated in detail, because they require the solving of the Boltzmann equation for a nonthermal atom population. This work estimates the effect of the UV radiation of the parent star and the accompanying photocleacton flux on the production of the suprathermal fraction of atomic hydrogen in the H2 → H transition region. We also consider the formation of the escaping flux of Hatoms created by this effect in the upper atmosphere of HD 209458b. We calculate the production rate and energy spectrum of the hydrogen atoms with excess kinetic energy during the dissociation of H2. Using the numerical stochastic model created by Shematovich (2004) for a hot planetary corona, we investigate the molecular-scale kinetics and transfer of suprathermal hydrogen atoms in the upper atmosphere and the emergent flux of atoms evaporating from the atmosphere. The latter is estimated as 3.4 × 1012 cm−2 s−1 for a moderate stellar activity level of UV radiation, which leads to a planetary atmosphere evaporation rate of 3.4 × 109 g s−1 due to the process of the dissociation of H2. This estimate is close to the observational value of ∼1010 g s−1 for the rate of atmospheric loss of HD 209458b.