[论文解读] Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) VI. Non-detection of sodium with HARPS on the bloated super-Neptune WASP-127b

J. V. Seidel, M. Lendl+18 more|arXiv (Cornell University)|Sep 28, 2020

Stellar, planetary, and galactic studies参考文献 62被引用 4

一句话总结

本研究使用HEARTS调查的HARPS数据，对高度 irradiated、蓬松的超级海王星WASP-127b进行了高分辨率凌星光谱的重新分析。结果表明，此前声称的5σ钠检测是假阳性结果，源于地球大气钠发射和信噪比较低的残差；真正的钠吸收深度为0.46% ± 0.20%（2.3σ），仅D2线可检测到，暗示可能存在异常的线强比或大气结构。

ABSTRACT

WASP-127b is one of the puffiest exoplanets found to date, with a mass only $3.4$ Neptune masses, but a radius larger than Jupiter. It is also located at the border of the Neptune desert, which describes the lack of highly-irradiated Neptune-sized planets, and which remains poorly understood. Its large scale height and bright host star make the transiting WASP-127b a valuable target to characterise in transmission spectroscopy. We use combined EulerCam and TESS light curves to recalculate the system's parameters. Additionally, we present an in-depth search for sodium in four transit observations of WASP-127b, obtained as part of the Hot Exoplanet Atmosphere Resolved with Transit Spectroscopy (HEARTS) survey with the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph. Two nights from this dataset were analysed independently by another team, claiming a detection of sodium incompatible with previous studies of data from both ground and space. We show that this large sodium detection is actually due to contamination from telluric sodium emissions and the low S/N in the core of the deep stellar sodium lines. When properly accounting for these effects, the previous sodium signal is reduced to an absorption of $0.46\pm0.20\%$ ($2.3σ$), which is compatible with analyses of WASP-127b transits carried out with other instruments. We can fit a Gaussian to the D2 line, however, the D1 line was not detected, indicating an unusual line ratio if sodium exists in the atmosphere. Follow-up of WASP-127 at both high-resolution and with high sensitivity will be required to firmly establish the presence of sodium and analyse its line shape.

研究动机与目标

• 使用高分辨率凌星光谱重新评估高度 irradiated、蓬松的超级海王星WASP-127b大气中钠的存在。
• 解决先前报道的5σ钠检测与早期低分辨率研究中未发现显著信号之间的矛盾。
• 评估地球大气污染和低信噪比残差对系外行星大气高分辨率光谱分析的影响。
• 利用HARPS数据，综合考虑仪器和大气系统误差，建立WASP-127b中钠吸收的可靠上限。
• 研究钠D1和D2线检测的不对称性，可能表明存在异常的大气线型或消光效应。

提出的方法

• 使用欧洲南方天文台3.6米望远镜上的HARPS光谱仪获取了四次高分辨率凌星观测数据，针对WASP-127b。
• 结合EulerCam和TESS的光变曲线，以优化系统参数，包括轨道倾角和凌星持续时间。
• 通过建模恒星钠线并校正地球大气污染，对透射光谱进行了详细分析。
• 考虑恒星钠线核心区域低信噪比残差的影响，这些残差可能模拟出大气吸收特征。
• 使用与HST/STIS和Spake等人（2019）一致的波长网格重新分析数据，以实现跨仪器比较。
• 计算出未来高分辨率研究中钠吸收深度的3σ上限为0.594%，为后续观测提供指导。

实验结果

研究问题

• RQ1 此前报道的WASP-127b中5σ钠检测是否由仪器或大气系统误差引起？
• RQ2 地球大气钠发射和恒星钠线中低信噪比残差在多大程度上影响高分辨率凌星光谱分析？
• RQ3 为何D2线可检测而D1线不可检测，尽管在热平衡大气中其预期强度比为2:1？
• RQ4 WASP-127b中测得的钠吸收与HST、斯皮策望远镜及地面望远镜先前的检测结果相比如何？
• RQ5 在纠正系统误差后，WASP-127b大气中钠吸收的真实幅度是多少？

主要发现

• 此前报道的WASP-127b中5σ钠检测是假阳性结果，由地球大气钠发射和恒星钠线中低信噪比残差引起。
• 在纠正系统误差后，真实的钠吸收深度为0.46% ± 0.20%，对应2.3σ检测，与其它研究中未检测到的结果一致。
• 仅钠双线中的D2线能用高斯轮廓拟合；D1线未被检测到，表明可能存在异常的线强比或大气结构。
• 数据与HST和斯皮策望远镜先前的低分辨率检测结果相容，但信号更弱，且在1σ水平上与未检测到的结果一致。
• 为未来高分辨率研究设定了0.594%的3σ钠吸收深度上限，为后续观测提供了关键基准。
• 本研究强调了在高分辨率系外行星透射光谱学中仔细建模系统误差的关键重要性，特别是针对具有强恒星谱线和明亮恒星的天体。