Abstract

We estimate the contribution of the molecular hydrogen dissociation processes, which are induced by the extreme UV radiation and the accompanying flux of photoelectrons, to the formation of a fraction of suprathermal atomic hydrogen in the Н₂ → Н transition region and the corresponding escape flux from the extended upper atmosphere of an exoplanet—the hot Neptune GJ 436b (Gliese 436b). We calculate the formation rate and the energy spectrum of hydrogen atoms, which are formed with the excess of kinetic energy when H₂ dissociates. With a numerical stochastic model of a hot planetary corona, we study at the molecular level the kinetics and transfer of suprathermal hydrogen atoms in the extended upper atmosphere and calculate the nonthermal escape flux. The escape flux has been estimated as 3.0 × 10¹² cm^–2 s^–1 for moderate activity of the star in the UV-radiation range, from which an upper estimate for the rate of the atmospheric loss caused by the H₂ dissociation processes has been found at 7.8 × 10⁸ g s^–1. The calculated value is within a range of approximately 3.7 × 10⁶ to 1.1 × 10⁹ g s^–1 for the estimates of the possible atmospheric loss rate obtained in observations of the exoplanet GJ 436b. The rate of the atmospheric loss due to suprathermal hydrogen atoms, which was estimated in calculations for the exoplanet GJ 436b, may be considered as a mean value, since the calculations were performed for the conditions corresponding to moderate UV-radiation activity of the star and the smallest values of the probabilities for predissociation of the excited electron levels of a H₂ molecule. We recommend this source of suprathermal hydrogen atoms to be included to the current aeronomic models of physical and chemical processes in the upper atmospheres of hot exoplanets.

中文翻译：

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H 2分解过程导致系外行星GJ 436b的非热大气损失

摘要

我们估计在分子氢离解过程，这是由极端紫外辐射和光电子的伴随磁通量感应的贡献，以超热原子氢在Н一小部分的形成₂ →Н过渡区域和从对应的逃逸磁通系外行星的扩展的上层大气-热的海王星GJ 436b（Gliese 436b）。我们计算了氢原子的形成速率和能谱，它们是在H ₂过量时动能形成的离解。利用热行星电晕的数值随机模型，我们在分子水平上研究了扩展的高层大气中超热氢原子的动力学和传递，并计算了非热逸出通量。对于恒星在紫外线辐射范围内的适度活动，逸出通量估计为3.0×10 ¹² cm ^–2 s ^–1，由此可以估算出由H ₂离解过程引起的大气损失速率的上限。被发现为7.8×10 ⁸ gs ^–1。计算值在大约3.7×10 ⁶到1.1×10 ⁹ gs ^{–1的范围内}用于对系外行星GJ 436b的观测中获得的可能的大气损失率的估计。在系外行星GJ 436b的计算中估算出的由超热氢原子引起的大气损失率可以认为是平均值，因为该计算是针对与恒星和恒星的中等UV辐射活性相对应的条件而进行的。 H ₂分子的激发电子能级预离解的概率的最小值。我们建议将这种超热氢原子源包括在当前热系外行星高层大气中的物理和化学过程的航空模型中。