[论文解读] The GAPS programme at TNG XXII. The GIARPS view of the extended helium atmosphere of HD189733 b accounting for stellar activity

G. Guilluy, V. Andretta+49 more|UA Campus Repository (The University of Arizona)|May 12, 2020

Stellar, planetary, and galactic studies参考文献 69被引用 12

一句话总结

本研究利用TNG望远镜上GIARPS仪器对HD 189733 b的扩展大气层进行同时的光学（Hα）与近红外（He I 1083.3 nm）光谱观测，以区分恒星活动效应与真实的氦吸收信号。研究发现，三次凌日观测显示氦吸收稳定在0.77 ± 0.04%，被解释为真实的行星大气信号；而其他凌日观测中信号的可变性则归因于恒星活动区与纤维结构，从而得以稳健地估算出质量损失率约为1–2 g s⁻¹。

ABSTRACT

Exoplanets orbiting very close to their host star are strongly irradiated. This can lead the upper atmospheric layers to expand and evaporate into space. The metastable helium (HeI) triplet at 1083.3nm has recently been shown to be a powerful diagnostic to probe extended and escaping exoplanetary atmosphere. We perform high-resolution transmission spectroscopy of the transiting hot Jupiter HD189733b with the GIARPS (GIANO-B + HARPS-N) observing mode of the Telescopio Nazionale Galileo, taking advantage of the simultaneous optical+near infrared spectral coverage to detect HeI in the planet's extended atmosphere and to gauge the impact of stellar magnetic activity on the planetary absorption signal. Observations were performed during five transit events of HD189733b. By comparison of the in- and out-of-transit GIANO-B observations we compute high-resolution transmission spectra, on which we perform equivalent width measurements and light-curves analyses to gauge the excess in-transit absorption in the HeI triplet. We detect an absorption signal during all five transits. The mean in-transit absorption depth amounts to 0.75+/-0.03%. We detect night-to-night variations in the HeI absorption signal likely due to the transit events occurring in presence of stellar surface inhomogeneities. We evaluate the impact of stellar-activity pseudo-signals on the true planetary absorption using a comparative analysis of the HeI and the H$α$ lines. We interpret the time-series of the HeI absorption lines in the three nights not affected by stellar contamination -exhibiting a mean in-transit absorption depth of 0.77+/-0.04%- using a 3-d atmospheric code. Our simulations suggest that the helium layers only fill part of the Roche lobe. Observations can be explained with a thermosphere heated to $\sim$12000 K, expanding up to $\sim$1.2 planetary radii, and losing $\sim$1 g/s of metastable helium.

研究动机与目标

• 在HD 189733 b的凌日光谱观测中，将真实的行星氦吸收信号与由恒星活动引起的伪信号相分离。
• 评估恒星表面非均质性（如亮斑与纤维结构）对近红外He I三重线测量的影响。
• 判断观测到的氦吸收变化是否源于恒星活动，还是行星大气本身的内在变化。
• 建立一种方法论，以在高分辨率凌日光谱中分离行星大气信号与恒星污染。

提出的方法

• 在HD 189733 b的多次凌日期间，对He I 1083.3 nm三重线与Hα线进行同步高分辨率光谱观测。
• 通过比较He I与Hα线轮廓，识别与恒星活动特征相关的相关性或反相关性。
• 利用EVE代码进行三维数值模拟，以模拟热层并根据观测到的吸收深度推断质量损失率。
• 识别与恒星纤维结构或活动区掩食相关的特定凌日信号（如发射峰或反相关行为）。
• 应用统计框架以区分行星大气吸收与由表面不均一性引起的恒星伪信号。
• 利用档案数据与同期的恒星活动诊断参数，验证瞬态信号的解释。

实验结果

研究问题

• RQ1 恒星活动区与表面不均一性在多大程度上会模仿或扭曲系外行星凌日光谱中的氦吸收信号？
• RQ2 凌日过程中Hα与He I 1083.3 nm线轮廓的变化如何与恒星表面特征相关联？
• RQ3 观测到的He I吸收是否可归因于真实的行星大气逃逸，还是受到恒星活动的污染？
• RQ4 在修正恒星活动效应后，HD 189733 b的真实质量损失率是多少？
• RQ5 恒星纤维结构与亮斑如何影响凌日系外行星近红外氦三重线吸收的解释？

主要发现

• 三次凌日（2、4和5号）显示恒星氦吸收稳定在0.77 ± 0.04%，被解释为不受恒星活动影响的稳健行星大气信号。
• 一次凌日（3号）显示He I与Hα信号呈反相关，解释为行星掩食了一个未完全覆盖的恒星活动区（亮斑）。
• 第一次凌日中，He I与Hα同时出现瞬态发射峰，归因于恒星纤维结构的掩食，而非大气吸收。
• 观测到的氦吸收最合理的解释是：一个被加热至约12,000 K的紧凑热层，其延伸范围可达罗氏洛球内1.2倍行星半径。
• 基于稳定吸收信号推导出的质量损失率约为1–2 g s⁻¹，与3D EVE模拟及先前研究结果一致。
• 恒星风与XUV辐射变异性无法解释观测到的氦信号变化，因为凌日时Hα的发射与风驱动吸收的预期不符。