14:15 〜 14:30
[PAE17-09] Hydrodynamic escape of H2-H2O atmospheres on terrestrial planets orbiting pre-main sequence M dwarfs
キーワード：流体力学的散逸、水素、水蒸気、地球型惑星、M型星、前主系列段階
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.
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.