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Nonthermal Atmospheric Loss of the Sub-Neptune π Men c Due to Exothermic Photochemistry


We estimated the contribution of reactions of exothermic photochemistry, namely, the dissociation of molecular hydrogen, by 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 of the extended upper atmosphere of an exoplanet—the hot sub-Neptune π Men c and the formation of the corresponding escape flux from the atmosphere. 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 2.5 × 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 Н2 dissociation processes has been found at 6.7 × 108 g s–1. The calculated value is close to the estimates of the possible atmospheric loss rate obtained in observations of the exoplanet π Men c in the range no higher than 1.0 × 109 g s–1. The rate of the atmospheric loss due to suprathermal hydrogen atoms, which was estimated in calculations for the exoplanet π Men c, 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 Н2 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.

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The work was supported by the Ministry of Science and Higher Education of the Russian Federation, project no. 075-15-2020-780, “Theoretical and experimental studies of the formation and evolution of extrasolar planetary systems and characteristics of exoplanets.”

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Correspondence to V. I. Shematovich.

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Avtaeva, A.A., Shematovich, V.I. Nonthermal Atmospheric Loss of the Sub-Neptune π Men c Due to Exothermic Photochemistry. Sol Syst Res 56, 67–75 (2022).

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