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Less Effective Hydrodynamic Escape of H2-H2O Atmospheres on Terrestrial Planets Orbiting Pre-main-sequence M Dwarfs

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Title: Less Effective Hydrodynamic Escape of H2-H2O Atmospheres on Terrestrial Planets Orbiting Pre-main-sequence M Dwarfs
Authors: Yoshida, Tatsuya Browse this author
Terada, Naoki Browse this author
Ikoma, Masahiro Browse this author
Kuramoto, Kiyoshi Browse this author →KAKEN DB
Issue Date: 2-Aug-2022
Publisher: IOP Publishing
Journal Title: The Astrophysical journal
Volume: 934
Issue: 2
Start Page: 137
Publisher DOI: 10.3847/1538-4357/ac7be7
Abstract: Terrestrial planets currently in the habitable zones around M dwarfs likely experienced a long-term runaway-greenhouse condition because of a slow decline in host-star luminosity in its pre-main-sequence phase. Accordingly, they might have lost significant portions of their atmospheres including water vapor at high concentration by hydrodynamic escape induced by the strong stellar X-ray and extreme ultraviolet (XUV) irradiation. However, the atmospheric escape rates remain highly uncertain due partly to a lack of understanding of the effect of radiative cooling in the escape outflows. Here we carry out 1D hydrodynamic escape simulations for an H2-H2O atmosphere on a planet with mass of 1M (circle plus) considering radiative and chemical processes to estimate the atmospheric escape rate and follow the atmospheric evolution during the early runaway-greenhouse phase. We find that the atmospheric escape rate decreases with the basal H2O/H2 ratio due to the energy loss by the radiative cooling of H2O and chemical products such as OH and OH+: the escape rate of H2 becomes one order of magnitude smaller when the basal H2O/H2 = 0.1 than that of the pure hydrogen atmosphere. The timescale for H2 escape exceeds the duration of the early runaway-greenhouse phase, depending on the initial atmospheric amount and composition, indicating that H2 and H2O could be left behind after the end of the runaway-greenhouse phase. Our results suggest that temperate and reducing environments with oceans could be formed on some terrestrial planets around M dwarfs.
Type: article
Appears in Collections:理学院・理学研究院 (Graduate School of Science / Faculty of Science) > 雑誌発表論文等 (Peer-reviewed Journal Articles, etc)

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