[P-AE17] Exoplanets

[PAE17-09] Hydrodynamic escape of H₂-H₂O atmospheres on terrestrial planets orbiting pre-main sequence M dwarfs

*Tatsuya Yoshida¹, Naoki Terada¹, Masahiro Ikoma², Kiyoshi Kuramoto³ (1.Tohoku University, 2.National Astronomical Observatory of Japan, 3.Hokkaido University)

Keywords:hydrodynamic escape, hydrogen, water vapor, terrestrial planets, M dwarfs, pre-main sequence

Terrestrial planets in the habitable zones around M dwarfs experience a long-term runaway greenhouse condition due to the stars' extended pre-main sequence phase. Accordingly, they might have lost most of their atmospheres including water vapor at high concentration by hydrodynamic escape induced by the strong stellar XUV irradiation. However, the atmospheric escape rates remain highly uncertain due to the uncertainty of the effect of the radiative cooling in the escape outflows. Here we carry out 1-D hydrodynamic escape simulations considering radiative processes and chemical processes for H₂-H₂O atmospheres derived from impact degassing and gravitational capture of nebular gas to estimate the atmospheric escape rates and the atmospheric evolution during the early runaway phase.

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 H₃⁺: 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 condition depending on the initial atmospheric amount and composition, indicating that H₂ and H₂O could be left behind after the end of the runaway phase. Our results suggest that moderate and reduced environments with ocean could be formed on terrestrial planets around M dwarfs.