11:00 〜 13:00
[PAE17-P02] Study of atmospheric ion escape from exoplanet TOI-700 d based on global multi-species MHD simulations
キーワード：系外惑星、大気散逸、MHDシミュレーション
One of the most important factors for habitability is the presence of an atmosphere, which is not only necessary to retain liquid water on the surface, but also to protect life from high-energy particles and radiation. The atmospheric escape from a planet can be greatly affected by existence of a planetary intrinsic magnetic field. For example, a strong dipole intrinsic magnetic field of ancient Mars may have reduced the atmospheric ion escape rate (Sakata et al., JGR, 2020). There have been many studies which focus on atmospheric escape from planets in our solar system. However, space environments around exoplanets can be very different from those around Earth.
In this study, we focused on exoplanet TOI 700 d which was discovered in January 2020 (Gilbert et al., AJ, 2020; Rodriguez et al., AJ, 2020). This is the first Earth-sized planet in the habitable zone (HZ) discovered by the Transiting Exoplanet Survey Satellite (TESS). The host star is a M dwarf star, which has lower surface temperature, thus closer HZ to the host star, and stronger X-ray and EUV (XUV) radiation in HZ than the solar system around a G-type star. Another important difference is that direction of the interplanetary magnetic field (IMF) around the planet may be dominated by the radial component because of the proximity to the host star and planet. In this study, we investigated how XUV flux, IMF orientation and intrinsic magnetic field affect atmospheric ion escape.
To model the space environment around TOI-700 d, we used multi-species MHD simulations model, REPPU-Planets (e.g., Terada et al., JGR, 2009; Sakata et al., JGR, 2020). Our model solved three-dimensional multispecies MHD equations including continuity equations for 11 ion species (O+, O2+, CO2+, NO+, CO+, N2+, N+, C+, He+, H+, Ar+) from the bottom of the ionosphere to the inter-planetary space where a constant stellar wind is assumed. It includes photoionization, electron impact ionization, charge exchange, ion-neutral reactions, dissociative recombination, and collisions (ion-electron, ion-neutral, electron-neutral). As stellar wind conditions, number density, velocity, and temperature were set to 450 cm-3, 470 km s-1, and 1.3× 106 K, respectively, by referring to previous studies (Cohen et al., ApJ, 2020; Dong et al., ApJL, 2020). IMF was assumed to be a Parker spiral with an angle either of 4° or 45° degrees and a magnitude of 12 nT. Also, the stellar XUV flux was set between 1 and 50 times of the current Earth value. We assumed a Venus-like atmospheric composition that depends on the stellar XUV flux as the input neutral atmosphere based on Kulikov et al. (SSR, 2007). In general, there is no information on the intrinsic magnetic fields of exoplanets. In this study, we assumed that the intrinsic magnetic field is either not exist or the global dipole magnetic field with the dipole moment perpendicular to the ecliptic plane of the stellar system. In the latter magnetized case, the equatorial surface strength of the dipole magnetic field was set to 1000 nT, which is strong enough to deflect the stellar wind and expected to reduce the atmospheric ion escape rate (Sakata et al., JGR, 2020). Simulation results suggest that even if the atmosphere of TOI-700 d is Venus-like, it will be difficult to retain it for a long time under strong XUV condition above 25 times of the current Earth, when the exoplanet does not possess the intrinsic magnetic field. The existence of the intrinsic magnetic field reduces the escape rate and helps the exoplanet to retain its atmosphere. This study also revealed that the escape rate of atmospheric ions, especially molecular ions, can be suppressed by small IMF Parker spiral angle which may be typical as the planet and the host star get closer. In the presentation, effects of direction of the IMF and intrinsic magnetic field on the ion escape mechanisms will be also reported in detail.
In this study, we focused on exoplanet TOI 700 d which was discovered in January 2020 (Gilbert et al., AJ, 2020; Rodriguez et al., AJ, 2020). This is the first Earth-sized planet in the habitable zone (HZ) discovered by the Transiting Exoplanet Survey Satellite (TESS). The host star is a M dwarf star, which has lower surface temperature, thus closer HZ to the host star, and stronger X-ray and EUV (XUV) radiation in HZ than the solar system around a G-type star. Another important difference is that direction of the interplanetary magnetic field (IMF) around the planet may be dominated by the radial component because of the proximity to the host star and planet. In this study, we investigated how XUV flux, IMF orientation and intrinsic magnetic field affect atmospheric ion escape.
To model the space environment around TOI-700 d, we used multi-species MHD simulations model, REPPU-Planets (e.g., Terada et al., JGR, 2009; Sakata et al., JGR, 2020). Our model solved three-dimensional multispecies MHD equations including continuity equations for 11 ion species (O+, O2+, CO2+, NO+, CO+, N2+, N+, C+, He+, H+, Ar+) from the bottom of the ionosphere to the inter-planetary space where a constant stellar wind is assumed. It includes photoionization, electron impact ionization, charge exchange, ion-neutral reactions, dissociative recombination, and collisions (ion-electron, ion-neutral, electron-neutral). As stellar wind conditions, number density, velocity, and temperature were set to 450 cm-3, 470 km s-1, and 1.3× 106 K, respectively, by referring to previous studies (Cohen et al., ApJ, 2020; Dong et al., ApJL, 2020). IMF was assumed to be a Parker spiral with an angle either of 4° or 45° degrees and a magnitude of 12 nT. Also, the stellar XUV flux was set between 1 and 50 times of the current Earth value. We assumed a Venus-like atmospheric composition that depends on the stellar XUV flux as the input neutral atmosphere based on Kulikov et al. (SSR, 2007). In general, there is no information on the intrinsic magnetic fields of exoplanets. In this study, we assumed that the intrinsic magnetic field is either not exist or the global dipole magnetic field with the dipole moment perpendicular to the ecliptic plane of the stellar system. In the latter magnetized case, the equatorial surface strength of the dipole magnetic field was set to 1000 nT, which is strong enough to deflect the stellar wind and expected to reduce the atmospheric ion escape rate (Sakata et al., JGR, 2020). Simulation results suggest that even if the atmosphere of TOI-700 d is Venus-like, it will be difficult to retain it for a long time under strong XUV condition above 25 times of the current Earth, when the exoplanet does not possess the intrinsic magnetic field. The existence of the intrinsic magnetic field reduces the escape rate and helps the exoplanet to retain its atmosphere. This study also revealed that the escape rate of atmospheric ions, especially molecular ions, can be suppressed by small IMF Parker spiral angle which may be typical as the planet and the host star get closer. In the presentation, effects of direction of the IMF and intrinsic magnetic field on the ion escape mechanisms will be also reported in detail.