14:30 〜 14:45
[PEM11-14] Effects of stellar spectra on atmospheric escape from a Mars-like planet orbiting inactive low-mass stars
キーワード：系外惑星、火星、大気散逸、惑星ー太陽風相互作用
Atmospheric evolution is one of the key parameters to understand how Earth and other planets were able to maintain thick atmosphere and habitability. In particular, atmospheric escape is strongly linked to atmospheric evolution. It is dependent on the planetary size, the existence of an intrinsic magnetic field, its intensity, on the stellar activity and stellar wind conditions. The variation in stellar activity affects both thermal escape and nonthermal escape. Ancient Mars, when the solar activity was more active than at present, had a very high atmospheric escape rate, the magnitude of the X-ray and extreme ultraviolet (XUV) irradiance being one of the parameters to determine the ion escape rate.
Many exoplanets have been discovered in recent years, and among them, M and K dwarfs are of particular interest because they might have habitable environments. The habitable zones of these stars are located very close to the main stars within 0.1 AU. Exoplanets in the habitable zones of these stars must therefore be exposed to intense XUV radiation and stellar winds. Numerical simulations suggested that ion escape rate from Proxima Centauri b, orbiting at 0.049 AU, is three orders of magnitude greater than at the present Mars, with a value of ~1027 s-1 (Dong et al., 2017). The previous study assumed an XUV intensity several tens of times that of the Sun to determine ion escape rate, but in fact the shape of the XUV spectrum determines the thermospheric profile, controlling in the ion escape rate.
This study presents the influences of stellar XUV spectrum on ion escape using a terrestrial exoplanetary thermosphere model and a multispecies magnetohydrodynamic model (REPPU-Planets). The target stellar systems are Sun, HD85512, and GJ581. Note that these stars are somewhat inactive XUV environments compared to Proxima Centauri. The planets are of Mars type and located at 1.524, 0.622, and 0.174 AU, respectively, in order to keep the same irradiance as that of the Martian orbit in the present solar system. According to thermosphere model, the thermosphere of the HD85512 system is the most extended, followed by the GJ581 system. The ion escape rates are estimated by REPPU-Planets simulations using these thermospheric profiles as input. Ion escape is the most intense for the HD85512 system, followed by Sun/Mars despite the weakest XUV radiation. In this presentation, the escape mechanism is discussed by comparing the XUV intensity at different wavelengths.
Many exoplanets have been discovered in recent years, and among them, M and K dwarfs are of particular interest because they might have habitable environments. The habitable zones of these stars are located very close to the main stars within 0.1 AU. Exoplanets in the habitable zones of these stars must therefore be exposed to intense XUV radiation and stellar winds. Numerical simulations suggested that ion escape rate from Proxima Centauri b, orbiting at 0.049 AU, is three orders of magnitude greater than at the present Mars, with a value of ~1027 s-1 (Dong et al., 2017). The previous study assumed an XUV intensity several tens of times that of the Sun to determine ion escape rate, but in fact the shape of the XUV spectrum determines the thermospheric profile, controlling in the ion escape rate.
This study presents the influences of stellar XUV spectrum on ion escape using a terrestrial exoplanetary thermosphere model and a multispecies magnetohydrodynamic model (REPPU-Planets). The target stellar systems are Sun, HD85512, and GJ581. Note that these stars are somewhat inactive XUV environments compared to Proxima Centauri. The planets are of Mars type and located at 1.524, 0.622, and 0.174 AU, respectively, in order to keep the same irradiance as that of the Martian orbit in the present solar system. According to thermosphere model, the thermosphere of the HD85512 system is the most extended, followed by the GJ581 system. The ion escape rates are estimated by REPPU-Planets simulations using these thermospheric profiles as input. Ion escape is the most intense for the HD85512 system, followed by Sun/Mars despite the weakest XUV radiation. In this presentation, the escape mechanism is discussed by comparing the XUV intensity at different wavelengths.