New observations of the extended hydrogen exosphere of the extrasolar planet HD 209458b
New observations of the extended hydrogen exosphere of the extrasolar planet HD 209458b
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对太阳系外行星HD 209458b扩展氢外逸层的新观测
DOI：
10.1051/0004-6361:200809460
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发表时间：
2008
影响因子：
6.5
通讯作者：
G. Ballester
中科院分区：
文献类型：
--
作者：
D. Ehrenreich;A. D. Étangs;G. H'ebrard;J. D'esert;A. Vidal;J. McConnell;C. Parkinson;G. Ballester
背景：从 HD 209458b 行星逸出的原子氢提供了迄今为止检测到的太阳系外行星大气层的最大观测特征。然而，之前观测研究中使用的太空望远镜成像摄谱仪（STIS）已不再可用，而仍然需要额外的观测来更好地约束蒸发过程的机制并确定其他“热木星”的蒸发状态。目标：在这里，我们的目标是利用哈勃太空望远镜 (HST) 上的高级巡天相机 (ACS) 探测 HD 209458b 扩展的氢外逸层，并找到拖曳行星的氢彗星状尾部的证据，尾部的大小取决于逃逸率和恒星发出的电离辐射量。这些观测结果还为其他凌日行星在近距离巨行星蒸发状态的比较研究框架内提供了基准。方法：使用 8 个 HST 轨道来观测 HD 209458b 的两次凌日。凌日光曲线是通过在两次凌日期间对未解析的恒星莱曼-α (Lyα) 发射线进行光度测定获得的。将凌日期间外层氢的吸收特征与预测氢尾的光曲线模型进行比较。结果：在访视 1 和访视 2 中，在整个 Lyα 线上测量到的穿越深度分别为（9.6±7.0）％和（5.3±10.0）％。对两次访问的数据进行平均，我们发现吸收深度为（8.0±5.7）％，与之前的研究非常一致。结论：外逸层的扩大证实了地球很可能正在向太空失去氢，然而，所达到的光度精度不允许我们更好地限制氢的质量损失率。基于哈勃太空望远镜上的高级巡天相机进行的观测。
Context: Atomic hydrogen escaping from the planet HD 209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. However, the Space Telescope Imaging Spectrograph (STIS) used in previous observational studies is no longer available, whereas additional observations are still needed to better constrain the mechanisms subtending the evaporation process and to determine the evaporation state of other “hot Jupiters”. Aims: Here, we aim to detect the extended hydrogen exosphere of HD 209458b with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find evidence of a hydrogen comet-like tail trailing the planet, whose size would depend on the escape rate and the amount of ionizing radiation emitted by the star. These observations also provide a benchmark for other transiting planets, in the frame of a comparative study of the evaporation state of close-in giant planets. Methods: Eight HST orbits were used to observe two transits of HD 209458b. Transit light curves were obtained by performing photometry of the unresolved stellar Lyman-α (Lyα) emission line during both transits. Absorption signatures of exospheric hydrogen during the transit were compared to light curve models predicting a hydrogen tail. Results: Transit depths of (9.6±7.0)% and (5.3±10.0)% were measured on the whole Lyα line in visits 1 and 2, respectively. Averaging data from both visits, we find an absorption depth of (8.0±5.7)%, in good agreement with previous studies. Conclusions: The extended size of the exosphere confirms that the planet is most likely losing hydrogen to space, yet, the photometric precision achieved does not allow us to better constrain the hydrogen mass-loss rate. Based on observations made with the Advanced Camera for Surveys on board the Hubble Space Telescope.