16:30 〜 16:45
[PAE18-23] The atmospheric particle simulations for observations of the upper atmosphere of terrestrial exoplanets by ultraviolet transit spectroscopy
キーワード：系外地球型惑星、紫外線分光、高層大気、暴走温室効果
Many terrestrial planets located around the habitable zone (HZ) have been discovered. Whether these planets are habitable or not is one of the most important issues in exoplanetary science. While JWST has advanced our observational understanding about their atmospheres of gas giants, we have little knowledge of terrestrial planets yet because of their small radius and scale height.
The major targets of terrestrial exoplanets are planets around low-mass stars at present. Since Low-mass stars are activie, the intensity of X-ray and extreme ultraviolet (XUV) radiation for planets in HZ are much stronger than that of the present-day Earth although the intensity of total radiation is comparable to that of what the present-day Earth. Thus, the terrestrial upper atmosphere is likely to obtain enough energy to enhance the atmospheric escape. On the other hand, the planet orbiting inner edge of HZ would have a water-vapor atmosphere in a runaway greenhouse state when terrestrial planets have a large amount of H2O like the Earth. Therefore, the major composition of the upper atmosphere becomes hydrogen and oxygen due to photodissociation of H2O. Such a hydrogen-rich upper atmosphere would be in the hydrodynamic escape heated by strong XUV radiation. In practice, a large transit depth in the blue wing of the Ly-α emission line was detected due to escaping hydrogen atmospheres accelerated by radiation pressure in highly irradiated hydrogen-dominated gas giants. In addition, the central part of the Ly-α line is absorbed by interstellar medium, but the dimming in the blue wing is not affected by interstellar absorption. Thus, there is a feasibility that the terrestrial upper atmosphere in the runaway greenhouse state could be detected through observations of the Ly-α line. Since transit features strongly depend on the atmospheric escape rate, ultraviolet spectroscopy could give new insights on atmospheric evolution of terrestrial atmospheres. Therefore, it is necessary to model the hydrogen-rich upper atmosphere in the runaway greenhouse state and investigate the feasibility of observing hydrodynamic escaping terrestrial atmospheres.
In this study, we estimate the Ly-α light curves for planets in runaway greenhouse state using a 3D atmospheric particle model based on Bourrier et al. (2013). We adopt the planetary parameters of TRAPPIST-1d which is an example of a planet where a runaway greenhouse would occur. In this model, the orbital motion of the planet and the trajectories of the escaping hydrogen atoms are calculated in the rest frame of the host star. In the motion of hydrogen atoms, we consider stellar and planetary gravity in addition to the radiation pressure from the Lyα line, assuming that hydrogen atoms are collision less. The photoionization of hydrogen and the charge exchange due to interaction with the stellar wind are included in this model as an effect of the reduction in the number of hydrogen atoms, but the recombination of ionized hydrogen is not considered. We will discuss the feasibility for observation in the upper atmospheres of terrestrial planets using Ly-α light curves, and discuss detectable atmospheric conditions, including mass-loss rates.
The major targets of terrestrial exoplanets are planets around low-mass stars at present. Since Low-mass stars are activie, the intensity of X-ray and extreme ultraviolet (XUV) radiation for planets in HZ are much stronger than that of the present-day Earth although the intensity of total radiation is comparable to that of what the present-day Earth. Thus, the terrestrial upper atmosphere is likely to obtain enough energy to enhance the atmospheric escape. On the other hand, the planet orbiting inner edge of HZ would have a water-vapor atmosphere in a runaway greenhouse state when terrestrial planets have a large amount of H2O like the Earth. Therefore, the major composition of the upper atmosphere becomes hydrogen and oxygen due to photodissociation of H2O. Such a hydrogen-rich upper atmosphere would be in the hydrodynamic escape heated by strong XUV radiation. In practice, a large transit depth in the blue wing of the Ly-α emission line was detected due to escaping hydrogen atmospheres accelerated by radiation pressure in highly irradiated hydrogen-dominated gas giants. In addition, the central part of the Ly-α line is absorbed by interstellar medium, but the dimming in the blue wing is not affected by interstellar absorption. Thus, there is a feasibility that the terrestrial upper atmosphere in the runaway greenhouse state could be detected through observations of the Ly-α line. Since transit features strongly depend on the atmospheric escape rate, ultraviolet spectroscopy could give new insights on atmospheric evolution of terrestrial atmospheres. Therefore, it is necessary to model the hydrogen-rich upper atmosphere in the runaway greenhouse state and investigate the feasibility of observing hydrodynamic escaping terrestrial atmospheres.
In this study, we estimate the Ly-α light curves for planets in runaway greenhouse state using a 3D atmospheric particle model based on Bourrier et al. (2013). We adopt the planetary parameters of TRAPPIST-1d which is an example of a planet where a runaway greenhouse would occur. In this model, the orbital motion of the planet and the trajectories of the escaping hydrogen atoms are calculated in the rest frame of the host star. In the motion of hydrogen atoms, we consider stellar and planetary gravity in addition to the radiation pressure from the Lyα line, assuming that hydrogen atoms are collision less. The photoionization of hydrogen and the charge exchange due to interaction with the stellar wind are included in this model as an effect of the reduction in the number of hydrogen atoms, but the recombination of ionized hydrogen is not considered. We will discuss the feasibility for observation in the upper atmospheres of terrestrial planets using Ly-α light curves, and discuss detectable atmospheric conditions, including mass-loss rates.