| 其他摘要 | The atmospheric escape of exoplanets has a crucial influence on the planetary composition,evolution and habitability. It is a frontier and hot issue in the modern astronomy and astrophysics. Transmission spectroscopy is a powerful method for detecting the planetary atmosphere. Escaping atmosphere has been detected by the excess absorption of spectral lines at different wavelengths during transits. The escaping atmosphere of hydrogen driven by stellar X-ray and extreme Ultraviolet (XUV) has been detected around some exoplanets by analysing the excess absorption of Ly𝛼 in far ultraviolet band. In the optical band the excess absorption of H𝛼 has also been detected. However, the current models that can explain the H𝛼 signals are very limited. Moreover,the helium infrared triplet line He 10830 is an important probe to detect helium atmosphere around exoplanets. Because the stellar Ly𝛼 is affected by the absorption of the interstellar medium and the Ly𝛼 emission from geocorona, it can only be observed with space-borne telescopes. Both the H𝛼 and He 10830 lines, however, cannot be contaminated by the interstellar medium and can be detected with the ground-based instrumentations. Up to now, in some systems both the absorption lines have been detected. Simultaneously modeling the absorption of the H𝛼 and He 10830 lines can provide useful constraints on the exoplanetary atmosphere. However, such work is very rare. In this thesis, we present our works to investigate the above issues. First, we as an improvement, a new Monte Carlo model with assumptions of a spherical stellar Ly𝛼 radiation and a spherical planetary atmosphere is introduced for the first time. Following are the main results: For WASP-121b, our fiducial model predicts a mass loss rate of ∼ 1.28×1012 g s−1 for this planet. Due to the high temperature and Ly𝛼 intensity predicted by the fiducial model, many hydrogen atoms are populated into the first excited state. As a consequence,the transmission spectrum of H𝛼 simulated by our model is broadly consistent with the observation. Comparing with the absorption of H𝛼 at different observation times, the stellar XUV flux varies in the range of 0.5-1.5 times the fiducial value, which may reflect the variation of the stellar activity. Finally, we find that the supersonic regions of the planetary wind contribute a prominent portion to the absorption of H𝛼 by comparing the equivalent width, which hints that a transonic outflow of the upper atmosphere driven by XUV irradiation of the host star can be detected by the ground-based telescope and the H𝛼 can be a good indicator of escaping atmosphere. For WASP-52b, through modeling the transmission spectra of the H𝛼 and He 10830 lines, we find that models with many different hydrogen to helium abundance ratios can reproduce the H𝛼 observations well if the host star has a high XUV flux and a relatively low X-ray fraction in XUV radiation, or a low XUV flux and a high X-ray fraction. The simulations of He 10830 triplet suggest that a higher hydrogen to helium abundance ratio than the solar value is required to fit the observation. Finally, the models that fit both lines well confine hydrogen to helium abundance ratio to be ∼ 98/2, XUV flux to be about 0.5 times the fiducial value, and the X-ray fraction in XUV radiation to have a value around 30%. The models also suggest that hydrogen and helium originate from the escaping atmosphere, and the mass-loss rate is about 2.8×1011 g s−1. Such an escaping atmosphere may affect the evolution of planetary atmosphere. This work is helpful for the future detection of the escaping planetary atmosphere of hydrogen and helium by using the ground-based telescopes. It provides a valuable reference for the research of planetary atmosphere and would be a benchmark in the study of planetary atmosphere. With the development of ground-based and space-based telescopes, it is possible to explore more of the unknown atmosphere. In the future, we will explain more observations through theoretical models and systematically explore the internal patterns of planetary atmospheric escape in combination with planetary evolution. Such work could not only help us with a deeper understanding of the escape and evolution of exoplanetary atmosphere, but also provide a valuable reference for the study of the escape of exoplanetary atmospheres in the early solar system. |
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