14:45 〜 15:00
[PEM11-15] 系外惑星TOI-700 dからの電離大気散逸の研究
キーワード：大気散逸、MHDシミュレーション、M型星
The recent discovery of Earth-size planets in the habitable zones (HZ) of M dwarfs has focused attention on whether liquid water and life exist on these planets. M dwarfs have lower temperatures and luminosities, and their HZ is much closer to the host star than Sun-like stars. Terrestrial planets around M dwarfs in the HZ are relatively easy to observe because of frequent transits and their relatively large ratio of cross-sectional areas of the planet to the host star. Since close-in exoplanets are also suitable for characterizing their atmospheres using transmission spectroscopy methods, exoplanets around M dwarfs provide our first opportunity to seek potentially habitable worlds. However, close-in exoplanets around M dwarfs are expected to experience strong ion loss because the X-ray and EUV (XUV) radiation from M dwarfs is substantially larger than that from Sun-like stars.
We focused here on exoplanet TOI-700 d, which was discovered in 2020 (Gilbert et al., AJ, 2020; Rodriguez et al., AJ, 2020). This is the first Earth-size planet in the HZ of M dwarf discovered by the Transiting Exoplanet Survey Satellite (TESS). In this study, we assessed the feasibility for atmospheric retention on a Venus-like exoplanet at the TOI-700 d location using multi-species MHD simulations model, REPPU-Planets (Terada et al., JGR, 2009; Sakata et al., 2022). Simulations were conducted under different conditions for the interplanetary magnetic field (IMF) orientation, the planetary intrinsic magnetic field, and the XUV radiation to investigate their effects on atmospheric escape. We assumed a Venus-like atmospheric composition that depends on the stellar XUV flux as the input neutral atmosphere (Kulikov et al., SSR, 2007). The similar stellar wind conditions to previous studies (Dong et al., ApJL, 2020) were used in the simulations.
In unmagnetized cases, the total escape rate increases with increasing XUV flux because the stronger XUV flux results in higher exospheric temperatures and an expanded neutral upper atmosphere. As IMF Parker spiral angle increases from 4 to 45 degrees, the escape rate of molecular ions (O2+ and CO2+) increases by an order of magnitude, while the O+ escape rate increases by a factor of 2. Also, the escape flux is confined to the meridional current sheet. This is because plasma in the meridional plane is efficiently accelerated by the magnetic tension force of the draped magnetic field and transported to the induced magnetotail. The effect of the IMF Parker spiral angle on the escape rate is stronger for molecular ions because they escape mainly from the ionosphere due to the lack of the corona originated source. In magnetized cases, the global intrinsic dipole magnetic field suppresses ion pickup loss from the neutral oxygen corona by deflecting the stellar wind at higher altitude than the unmagnetized cases. It reduces ion pickup loss, while promoting the cusp-origin escape from the low- altitude ionosphere. The strong intrinsic magnetic field suppresses the ion escape rate, while the weak intrinsic magnetic field slightly increases the ion escape rate.
The results also indicate that unmagnetized TOI-700 d would have difficulty to retain its atmosphere over a few billion years under strong XUV condition above 30 times of the current Earth. However, the dipole intrinsic magnetic field of 1000 nT at equatorial surface reduces the escape rate and enable the exoplanet to retain its atmosphere for a long time even under the strong XUV condition. It suggests that a global intrinsic magnetic field plays a crucial role to retain an atmosphere of exoplanets in the HZ around active M dwarfs.
We focused here on exoplanet TOI-700 d, which was discovered in 2020 (Gilbert et al., AJ, 2020; Rodriguez et al., AJ, 2020). This is the first Earth-size planet in the HZ of M dwarf discovered by the Transiting Exoplanet Survey Satellite (TESS). In this study, we assessed the feasibility for atmospheric retention on a Venus-like exoplanet at the TOI-700 d location using multi-species MHD simulations model, REPPU-Planets (Terada et al., JGR, 2009; Sakata et al., 2022). Simulations were conducted under different conditions for the interplanetary magnetic field (IMF) orientation, the planetary intrinsic magnetic field, and the XUV radiation to investigate their effects on atmospheric escape. We assumed a Venus-like atmospheric composition that depends on the stellar XUV flux as the input neutral atmosphere (Kulikov et al., SSR, 2007). The similar stellar wind conditions to previous studies (Dong et al., ApJL, 2020) were used in the simulations.
In unmagnetized cases, the total escape rate increases with increasing XUV flux because the stronger XUV flux results in higher exospheric temperatures and an expanded neutral upper atmosphere. As IMF Parker spiral angle increases from 4 to 45 degrees, the escape rate of molecular ions (O2+ and CO2+) increases by an order of magnitude, while the O+ escape rate increases by a factor of 2. Also, the escape flux is confined to the meridional current sheet. This is because plasma in the meridional plane is efficiently accelerated by the magnetic tension force of the draped magnetic field and transported to the induced magnetotail. The effect of the IMF Parker spiral angle on the escape rate is stronger for molecular ions because they escape mainly from the ionosphere due to the lack of the corona originated source. In magnetized cases, the global intrinsic dipole magnetic field suppresses ion pickup loss from the neutral oxygen corona by deflecting the stellar wind at higher altitude than the unmagnetized cases. It reduces ion pickup loss, while promoting the cusp-origin escape from the low- altitude ionosphere. The strong intrinsic magnetic field suppresses the ion escape rate, while the weak intrinsic magnetic field slightly increases the ion escape rate.
The results also indicate that unmagnetized TOI-700 d would have difficulty to retain its atmosphere over a few billion years under strong XUV condition above 30 times of the current Earth. However, the dipole intrinsic magnetic field of 1000 nT at equatorial surface reduces the escape rate and enable the exoplanet to retain its atmosphere for a long time even under the strong XUV condition. It suggests that a global intrinsic magnetic field plays a crucial role to retain an atmosphere of exoplanets in the HZ around active M dwarfs.