Nonthermal Atmospheric Loss of the Exoplanet GJ 436b due to H 2 Dissociation Processes

We estimate the contribution of the molecular hydrogen dissociation processes, which are induced by the extreme UV radiation and the accompanying flux of photoelectrons, to the formation of a fraction of suprathermal atomic hydrogen in the Н2 → Н transition region and the corresponding escape flux from the extended upper atmosphere of an exoplanet—the hot Neptune GJ 436b (Gliese 436b). We calculate the formation rate and the energy spectrum of hydrogen atoms, which are formed with the excess of kinetic energy when H2 dissociates. With a numerical stochastic model of a hot planetary corona, we study at the molecular level the kinetics and transfer of suprathermal hydrogen atoms in the extended upper atmosphere and calculate the nonthermal escape flux. The escape flux has been estimated as 3.0 × 1012 cm–2 s–1 for moderate activity of the star in the UV-radiation range, from which an upper estimate for the rate of the atmospheric loss caused by the H2 dissociation processes has been found at 7.8 × 108 g s–1. The calculated value is within a range of approximately 3.7 × 106 to 1.1 × 109 g s–1 for the estimates of the possible atmospheric loss rate obtained in observations of the exoplanet GJ 436b. The rate of the atmospheric loss due to suprathermal hydrogen atoms, which was estimated in calculations for the exoplanet GJ 436b, may be considered as a mean value, since the calculations were performed for the conditions corresponding to moderate UV-radiation activity of the star and the smallest values of the probabilities for predissociation of the excited electron levels of a H2 molecule. We recommend this source of suprathermal hydrogen atoms to be included to the current aeronomic models of physical and chemical processes in the upper atmospheres of hot exoplanets.