标题： HD 209458b逃逸大气中氦的质量分馏 显示英文标题

标题： The Mass Fractionation of Helium in the Escaping Atmosphere of HD 209458b

摘要： HD 209458b以及其它几个系外行星中亚稳态氦的吸收信号可以通过逃逸大气模型来解释，该模型假设其氦氢比低于太阳值。 如果它们原始大气中氦的含量较少，那么低氦丰度可能是行星形成的结果。 然而，另一种可能性是，低氦氢比是由大气中氦的质量分馏过程引起的。 为了研究氢-氦大气中分馏效应的影响，我们基于著名的开源MHD代码PLUTO开发了一个自洽的多流体一维流体力学模型。 我们的模拟表明，在多流体模型中可以自发产生较低的氦氢比。 我们进一步在广泛的参数空间内模拟了HD 209458b的氦10830线的传输光谱。 观测的传输光谱可以在相当于平静太阳1.8倍的X射线和极端紫外辐射通量条件下拟合。 同时，氦逃逸通量与氢逃逸通量之比，即$F_{He}/F_{H}$，为0.039。 我们的结果表明，氦相对于氢的质量分馏可以自然地解释观测所需的低氦氢比。 因此，在HD 209458b的逃逸大气中，即使其氦丰度类似于太阳，也不需要减少大气中的氦丰度。 本研究中的模拟提示，在逃逸大气中，质量分馏也可能发生在其他系外行星上，这需要进一步探索。 显示英文摘要

摘要： The absorption signals of metastable He in HD 209458b and several other exoplanets can be explained via escaping atmosphere model with a subsolar He/H ratio. The low abundance of helium can be a result of planet formation if there is a small amount of helium in their primordial atmosphere. However, another possibility is that the low He/H ratio is caused by the process of mass fractionation of helium in the atmosphere. In order to investigate the effect of the fractionation in the hydrogen-helium atmosphere, we developed a self-consistent multi-fluid 1D hydrodynamic model based on the well-known open-source MHD code PLUTO. Our simulations show that a lower He/H ratio can be produced spontaneously in the multi-fluid model. We further modeled the transmission spectra of He 10830 lines for HD 209458b in a broad parameter space. The transmission spectrum of the observation can be fitted in the condition of 1.80 times the X-ray and extreme-ultraviolet flux of the quiet Sun. Meanwhile, the ratio of the escaping flux of helium to hydrogen, $F_{He}/F_{H}$, is 0.039. Our results indicate that the mass fractionation of helium to hydrogen can naturally interpret the low He/H ratio required by the observation. Thus, in the escaping atmosphere of HD 209458b, decreasing the abundance of helium in the atmosphere is not needed even if its He abundance is similar to that of the Sun. The simulation presented in this work hints that in the escaping atmosphere, mass fractionation can also occur on other exoplanets, which needs to be explored further.