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Atmosphere Escape Inferred from Modeling the Hα Transmission Spectrum of WASP-121b
The Astrophysical Journal Letters ( IF 7.9 ) Pub Date : 2021-02-04 , DOI: 10.3847/2041-8213/abda41
Dongdong Yan 1, 2, 3 , Jianheng Guo 1, 2, 3 , Chenliang Huang 4 , Lei Xing 1, 2, 3
Affiliation  

The escaping atmospheres of hydrogen driven by stellar X-ray and extreme ultraviolet (XUV) have been detected around some exoplanets by the excess absorption of Lyα in the far-ultraviolet band. In the optical band the excess absorption of Hα is also found by ground-based instruments. However, it is not certain if the escape of the atmosphere driven by XUV can result in such absorption. Here we present the XUV-driven hydrodynamic simulation coupled with the calculation of detailed level population and the process of radiative transfer for WASP-121b. Our fiducial model predicts a mass-loss rate of ∼1.28 1012 g s−1 for WASP-121b. 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. Compared with the absorption of Hα at different observation times, the stellar XUV emission varies in the range of 0.5–1.5 times 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 of Hα, which hints that a transonic outflow of the upper atmosphere driven by XUV irradiation of the host star can be detected by a ground-based telescope and that Hα can be a good indicator of escaping atmosphere.



中文翻译:

气氛逃离建模为H推断α WASP-121b的透射谱

通过恒星X射线和极紫外(XUV)驱动氢的逸出大气压已经由Ly的过量吸收而检测出周围的一些外行星α在远紫外波段。在光带H的过量吸收α也被基于地面的文书中。但是,不确定由XUV驱动的大气逸出是否会导致这种吸收。在这里,我们介绍XUV驱动的水动力模拟,以及详细的水平面总体计算和WASP-121b的辐射传递过程。我们的基准模型预测WASP-121b的质量损失率约为1.28 10 12 gs -1。由于高温和Ly的α根据基准模型预测的强度,许多氢原子被填充到第一激发态。因此,H的透射谱α通过我们的模型模拟与观察大体一致。用H的吸收相比,α在不同的观察时间,恒星XUV发射在0.5-1.5倍基准值的范围内,这可能反映了恒星活动的变化而变化。最后,我们发现,所述行星风的超音速区有助于突出部H的吸收α通过比较H的等效宽度α,这暗示,由主机星XUV照射驱动的上部大气的跨音速流出可由基于地面的望远镜被检测和使得h α可以是逸出气氛的良好指标。

更新日期:2021-02-04
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