Japan Geoscience Union Meeting 2023

Presentation information

[E] Oral

P (Space and Planetary Sciences ) » P-EM Solar-Terrestrial Sciences, Space Electromagnetism & Space Environment

[P-EM11] Exoplanet

Wed. May 24, 2023 1:45 PM - 3:15 PM 102 (International Conference Hall, Makuhari Messe)

convener:Takanori Kodama(The university of Tokyo), Shota Notsu(Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research), Yui Kawashima(RIKEN), Mayuko Mori(The University of Tokyo), Chairperson:Akifumi Nakayama(Department of Physics, Rikkyo University), Takanori Kodama(The university of Tokyo)

2:30 PM - 2:45 PM

[PEM11-14] Effects of stellar spectra on atmospheric escape from a Mars-like planet orbiting inactive low-mass stars

*Shotaro Sakai1,2, Akifumi Nakayama3, Kanako Seki4, Naoki Terada1, Hiroyuki Shinagawa5, Ryoya Sakata4,1, François Leblanc6, David A. Brain7, Takashi Tanaka5,8 (1.Department of Geophysics, Graduate School of Science, Tohoku University, 2.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, 3.Department of Physics, College of Science, Rikkyo University, 4.Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 5.National Institute of Information and Communications Technology, 6.LATMOS/CNRS, Sorbonne Université, 7.Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, 8.International Research Center for Space and Planetary Environmental Science, Kyushu University)

Keywords:Exoplanets, Mars, Atmospheric escape, Stellar wind interaction with planets

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.