AI 中文总结
研究人员利用昴星团望远镜的IRD光谱仪，搜寻四颗围绕M型矮星运行的亚海王星与超级地球大气中的氦，未探测到显著氦吸收，得出相关置信上限，结合模型分析后提出其大气类型的可能解释，并建议未来观测其他吸收线。
AI 中文摘要
通过开普勒（Kepler）、凌日系外行星巡天卫星（TESS）等空间巡天项目，已发现数千颗亚海王星，它们的整体成分与内部结构被认为反映了其形成与演化路径，而大气观测可为这些过程提供约束。近红外氦三线是延伸的、正在逃逸的氢/氦大气的潜在示踪剂，近期包含几何效应的模型表明，围绕附近晚型M型矮星运行的行星可能为探测该信号提供有利条件。然而，目前仅报告发现了三颗围绕M型矮星运行的行星存在氦。我们利用昴星团望远镜（Subaru Telescope）上的红外多普勒（InfraRed Doppler, IRD）光谱仪，对围绕M型矮星运行的三颗亚海王星（TOI-2136b、TOI-654b、LP 791-18c）和一颗超级地球（TOI-1634b）开展了高分辨率透射光谱观测。我们在所有目标中均未发现具有统计显著性的氦吸收信号，因此得出各行星氦线深度的95%置信上限分别为：TOI-2136b 1.36%、TOI-1634b 0.60%、TOI-654b 2.07%、LP 791-18c 3.00%；等效宽度的95%置信上限分别为：TOI-2136b 7.3 mÅ、TOI-1634b 2.1 mÅ、TOI-654b 7.4 mÅ、LP 791-18c 9.1 mÅ。我们进一步将这些结果与等温帕克风模型对比，探索了对高层大气温度与质量损失率的约束；同时还对比了涵盖不同假设X射线光度的原始氢/氦大气的自洽ATES模型，发现假设的极紫外（XUV）通量变化无法解释此次未探测到氦的结果。该结果表明，这些行星拥有富金属的原始氢/氦大气，或非原始大气（如水富包层）。未来对其他吸收线（如莱曼α、氢α、水）的观测，或可为这些大气提供进一步约束。
英文摘要
Thousands of sub-Neptunes have been discovered mainly through space-based surveys such as Kepler and TESS. Their bulk compositions and internal structures are thought to reflect their formation and evolutionary pathways, and atmospheric observations provide constraints on these processes. The near-infrared helium triplet is a potential tracer of extended, escaping H/He atmospheres. Recent models that include geometric effects suggest that planets orbiting nearby late M dwarfs may offer favorable conditions for detecting this signal. Nevertheless, helium has been reported for only three planets around M dwarfs to date. We conducted high-resolution transmission spectroscopy of three sub-Neptunes (TOI-2136b, TOI-654b, and LP 791-18c) and a super-Earth (TOI-1634b) orbiting M dwarfs with the InfraRed Doppler (IRD) spectrograph on the Subaru Telescope. We find no statistically significant helium absorption in any target; accordingly, we derive 95% confidence upper limits on the helium line depth of 1.36%, 0.60%, 2.07%, and 3.00%, and on the equivalent width of 7.3, 2.1, 7.4, and 9.1 mÅ, for TOI-2136b, TOI-1634b, TOI-654b, and LP 791-18c, respectively. We further explored constraints on the upper-atmospheric temperature and mass-loss rate by comparing these results with isothermal Parker-wind models. While we have compared with self-consistent ATES models of primordial H/He atmospheres spanning a range of assumed X-ray luminosities, changes in the assumed XUV flux do not appear to account for the non-detections. The results suggest that these planets have metal-enriched H/He primary atmospheres or non-primordial atmospheres, such as water-rich envelopes. Future observations of other absorption lines, such as Lyman-$α$, H-$α$, and H$_2$O, may provide further constraints on these atmospheres.
Comments15 pages, 10 figures, accepted for publication in PASJ
