KELT-9b的原子碳氧比
The atomic C/O ratio of KELT-9b
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中文总结 AI 辅助
该研究通过MAROON-X光谱仪观测KELT-9b凌星，直接反演其原子C/O比为0.20，低于恒星和太阳值，并确认其大气热反转且略富金属，为行星形成提供约束。
中文摘要 AI 辅助
长期以来，理论认为巨行星大气中的碳氧比（C/O）蕴含可追溯至其形成历史的成分信息。通常，系外行星大气的C/O比是通过对CO、CO$_2$、H$_2$O和OH等主要含碳和含氧分子的丰度测量来推断的。然而，一些系外行星的温度极高，以至于在观测所探测的压力水平下，分子几乎可以完全热解离，这使得通过传统示踪物测量其C/O比变得困难。在此，我们不使用分子，而是旨在直接从其大气中探测到的原子碳（C）和氧（O）来反演KELT-9b（T$_\text{eq}=4000$K）的C/O比。我们分析了MAROON-X高分辨率光谱仪观测到的KELT-9b两次凌星数据，发现了来自原子C、O以及难熔金属的吸收交叉相关信号。由此，我们使用仅应用于受非局域热力学平衡效应影响较小的光谱区域的一维局域热力学平衡大气反演框架，推断了垂直温度结构、挥发性和难熔物种的相对比例以及KELT-9b的C/O比。我们测得KELT-9b大气的原子C/O比为$0.20_{-0.07}^{+0.13}$，略低于恒星值$0.38\pm0.15$，并显著低于太阳值$0.59\pm0.07$。此外，我们确认了先前对晨昏线区域的研究结果，发现KELT-9b的大气存在热反转且略富金属。我们测得挥发物与难熔物之比（冰岩比的替代指标）为$1.24_{-0.78}^{+1.93}$倍太阳值（$[M_{\text{vol}}/M_{\text{ref}}]=0.09_{-0.20}^{+0.19}$），这与KELT-9和太阳均一致。
英文摘要
The carbon-to-oxygen (C/O) ratio of a giant planet's atmosphere has long been theorised to hold compositional information that can be traced back to its formation history. Typically, the C/O ratio of an exoplanetary atmosphere is inferred from abundance measurements of major C- and O-bearing molecules such as CO, CO$_2$, H$_2$O, and OH. However, some exoplanets have such elevated temperatures that molecules can be nearly completely thermally dissociated at the pressure levels that observations probe, making it difficult to measure their C/O ratio via these traditional tracers. Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (T$_\mathrm{eq} = 4000$K) directly from atomic C and O, which are detected in its atmosphere. We analysed two transits of KELT-9b observed with the MAROON-X high-resolution spectrograph, finding absorption cross-correlation signals from atomic C and O as well as refractory metals. From this, we inferred the vertical temperature structure, the relative proportions of volatile and refractory species, and the C/O ratio of KELT-9b using a 1D local thermodynamic equilibrium atmospheric retrieval framework applied only to spectral regions mostly unaffected by non-local thermodynamic equilibrium effects. We measure the atomic C/O ratio of the atmosphere of KELT-9b to be $0.20_{-0.07}^{+0.13}$, which is slightly lower than the stellar value of $0.38\pm0.15$ and significantly below the solar value of $0.59\pm0.07$. We otherwise confirm previous investigations of the terminator region, finding KELT-9b's atmosphere to be thermally inverted and slightly metal-rich. We measure the volatile-to-refractory ratio, a proxy for the ice-to-rock ratio, to be $1.24_{-0.78}^{+1.93}$ $\times$ solar ($[$M$_{\mathrm{vol}}$/M$_{\mathrm{ref}}]$ = $0.09_{-0.20}^{+0.19}$), which is consistent with both KELT-9 and the Sun.
发表机构
- Observatoire astronomique de l’Université de Genève（日内瓦大学天文台）
- Univ Toulouse, CNES, CNRS, IRAP（图卢兹大学）
- School of Mathematical and Physical Sciences, Macquarie University（麦考瑞大学数学与物理科学学院）
- Lund Observatory, Division of Astrophysics, Department of Physics, Lund University（隆德大学天体物理系隆德天文台）
- Department of Astronomy & Astrophysics, University of Chicago（芝加哥大学天文与天体物理系）
- Gemini Observatory/NSF NOIRLab（吉米尼天文台/美国国家光学红外天文实验室）
- Landessternwarte, Zentrum für Astronomie der Universität Heidelberg（海德堡大学天文学中心州立天文台）
- Instituto de Astrofísica e Ciêccias do Espaço, Universidade do Porto（波尔图大学空间天体物理与科学研究所）
- Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto（波尔图大学理学院物理与天文系）
- Centre Vie dans l’Univers, Faculté des sciences de l’Université de Gen‘eve（日内瓦大学理学院宇宙生命研究中心）
- Space Research Institute, Austrian Academy of S（奥地利科学院太空研究所）
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