arXiv 2001.110482020-01-31astro-ph.EPastro-ph.HEastro-ph.SR

MOVES III。X射线和紫外线同时观测揭示了热木星HD 189733 b的可变环境

MOVES III. Simultaneous X-ray and ultraviolet observations unveiling the variable environment of the hot Jupiter HD 189733b

V. Bourrier, P. J. Wheatley, A. Lecavelier des Etangs, G. King, T. Louden, D. Ehrenreich, R. Fares, Ch. Helling, J. Llama, M. M. Jardine, A. A. Vidotto

备注：: 22 pages, 21 figures, accepted for publication in MNRAS on 23 January 2020
DOI：: 10.1093/mnras/staa256

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摘要：

在MOVES（多波长观测eVaporating Exoplanet及其恒星）计划的第三篇论文中，我们将哈勃太空望远镜的远紫外观测与XMM-牛顿/斯威夫特X射线观测相结合，以测量HD 189733在各种FUV谱线中的发射及其软X射线光谱。基于这些测量，我们将星际介质朝向HD 189733，并推导出该恒星的半合成XUV光谱，用于研究其高能辐射在五个不同时期的演化。观测到HD 189733的两个耀斑，但我们认为其光谱能量分布的长期变化对HD 189733 b的环境具有最重要的影响。日冕和风活动的减少可能有利于在行星前方的弓形激波中形成密集的Si $^{2+}$原子群，这是前两个时期测量的过境前和过境吸收的原因。在第二，第三和第五个时代，这可能是由于扩展的行星热层和恒星风质子通过与行星外逸层的电荷交换被中和的尾部，在莱曼- $α$线检测到过境吸收签名。我们建议，在行星的X射线照射的增加，并减少其EUV照射导致中性氢的光电离率较低，有利于检测这些签名，通过维持较高密度的H $^{0}$原子在高层大气中，并提高与恒星风的电荷交换。在最后一个纪元，更深和更广的吸收特征表明这颗行星进入了一个不同的蒸发状态，为恒星活动和行星环境结构之间的联系提供了线索。

英文摘要：

In this third paper of the MOVES (Multiwavelength Observations of an eVaporating Exoplanet and its Star) programme, we combine Hubble Space Telescope far-ultraviolet observations with XMM-Newton/Swift X-ray observations to measure the emission of HD 189733 in various FUV lines, and its soft X-ray spectrum. Based on these measurements we characterise the interstellar medium toward HD 189733 and derive semi-synthetic XUV spectra of the star, which are used to study the evolution of its high-energy emission at five different epochs. Two flares from HD 189733 are observed, but we propose that the long-term variations in its spectral energy distribution have the most important consequences for the environment of HD 189733b. Reduced coronal and wind activity could favour the formation of a dense population of Si$^{2+}$ atoms in a bow-shock ahead of the planet, responsible for pre- and in-transit absorption measured in the first two epochs. In-transit absorption signatures are detected in the Lyman-$α$ line in the second, third and fifth epochs, which could arise from the extended planetary thermosphere and a tail of stellar wind protons neutralised via charge-exchange with the planetary exosphere. We propose that increases in the X-ray irradiation of the planet, and decreases in its EUV irradiation causing lower photoionisation rates of neutral hydrogen, favour the detection of these signatures by sustaining larger densities of H$^{0}$ atoms in the upper atmosphere and boosting charge-exchanges with the stellar wind. Deeper and broader absorption signatures in the last epoch suggest that the planet entered a different evaporation regime, providing clues as to the link between stellar activity and the structure of the planetary environment.

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