Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b
Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b
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DOI：
10.1051/0004-6361/201731129
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发表时间：
2017-10
期刊：
影响因子：
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通讯作者：
J. Chadney;J. Chadney;T. Koskinen;M. Galand;Yvonne C. Unruh;J. Sanz-Forcada
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文献类型：
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作者：
J. Chadney;J. Chadney;T. Koskinen;M. Galand;Yvonne C. Unruh;J. Sanz-Forcada
恒星耀斑经常发生在年轻的低质量恒星上，许多被探测到的系外行星围绕着这些恒星运行。耀斑是能量充沛的脉冲事件，在解释凌日观测时需要考虑到它们对系外行星大气的影响。我们开发了一个模型来描述围绕耀斑恒星运行的太阳系外巨行星（EGPs）的上层大气。该模型模拟了近地egp高层大气的热逸。包括太阳辐射和电子冲击引起的电离，并模拟了光化学和扩散输运过程。这个模型被用来研究类似太阳的G星HD209458和年轻的K星HD189733对各自行星的耀斑影响。一个假想的类似hd209458b的行星绕着活跃的M恒星AU Mic运行，也被模拟出来。我们发现，典型耀斑对EGPs的中性高层大气影响不大。因此，恒星耀斑本身不会引起足够大的行星质量损失变化，以解释先前研究中看到的HD189733b过境深度的变化，尽管我们表明，极端的恒星质子事件可能导致所需的质量损失。然而，我们的模拟确实揭示了这些行星电离层中电子数密度的增强，其峰值位于恒星x射线被吸收的层中。电子密度达到耀斑前的2.2至3.5倍，增强的电子密度在耀斑开始后持续约3至10小时。耀斑的强度及其光谱能量分布的宽度影响电离增强的高度范围。在非常年轻的耀斑恒星（如AU - Mic）中，在耀斑光谱的XUV部分中有一个很大的宽带连续成分，导致围绕这颗恒星运行的行星的高度范围很广。
Stellar flares are a frequent occurrence on young low-mass stars around which many detected exoplanets orbit. Flares are energetic, impulsive events, and their impact on exoplanetary atmospheres needs to be taken into account when interpreting transit observations. We have developed a model to describe the upper atmosphere of Extrasolar Giant Planets (EGPs) orbiting flaring stars. The model simulates thermal escape from the upper atmospheres of close-in EGPs. Ionisation by solar radiation and electron impact is included and photochemical and diffusive transport processes are simulated. This model is used to study the effect of stellar flares from the solar-like G star HD209458 and the young K star HD189733 on their respective planets. A hypothetical HD209458b-like planet orbiting the active M star AU Mic is also simulated. We find that the neutral upper atmosphere of EGPs is not significantly affected by typical flares. Therefore, stellar flares alone would not cause large enough changes in planetary mass loss to explain the variations in HD189733b transit depth seen in previous studies, although we show that it may be possible that an extreme stellar proton event could result in the required mass loss. Our simulations do however reveal an enhancement in electron number density in the ionosphere of these planets, the peak of which is located in the layer where stellar X-rays are absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flare levels and enhanced electron densities last from about 3 to 10 hours after the onset of the flare. The strength of the flare and the width of its spectral energy distribution affect the range of altitudes that see enhancements in ionisation. A large broadband continuum component in the XUV portion of the flaring spectrum in very young flare stars, such as AU Mic, results in a broad range of altitudes affected in planets orbiting this star.