[论文解读] Connecting planet formation and astrochemistry: C/O and N/O of warm giant planets and Jupiter-analogs

A. Cridland, E. F. van Dishoeck+2 more|arXiv (Cornell University)|Sep 7, 2020

Astro and Planetary Science参考文献 88被引用 5

一句话总结

本研究通过原行星盘的化学演化模型，预测了温带气态巨行星（0.5–4 AU）的C/O与N/O比值，表明木星可能因化学成分不匹配而形成于数十AU之外。模型验证了固态吸积与大气C/O之间存在主序列关系，高金属度盘导致更大的化学离散度，并强调了碳难熔物侵蚀前沿位置在塑造行星大气中的关键作用。

ABSTRACT

(Abridged) The chemical composition of planetary atmospheres has long been thought to store information regarding where and when a planet accretes its material. Predicting this chemical composition theoretically is a crucial step in linking observational studies to the underlying physics that govern planet formation. As a follow-up to a study of hot Jupiters in our previous work, we present a population of warm Jupiters (semi-major axis between 0.5-4 AU) extracted from the same planetesimal formation population synthesis model as used in our previous work. We compute the astrochemical evolution of the protoplanetary disks included in this population to predict the carbon-to-oxygen (C/O) and nitrogen-to-oxygen (N/O) ratio evolution of the disk gas, ice, and refractory sources, the accretion of which greatly impacts the resulting C/O and N/O in the atmosphere of giant planets. We confirm that the main sequence (between accreted solid mass and atmospheric C/O) we found previously is largely reproduced by the presented population of synthetic warm Jupiters. And as a result, the majority of the population fall along the empirically derived mass-metallicity relation when the natal disk has solar or lower metallicity. Planets forming from disks with high metallicity ([Fe/H] $>$ 0.1) result in more scatter in chemical properties which could explain some of the scatter found in the mass-metallicity relation. Combining predicted C/O and N/O ratios shows that Jupiter does not fall among our population of synthetic planets, suggesting that it likely did not form in the inner 5 AU of the solar system before proceeding into a Grand Tack. This result is consistent with recent analysis of the chemical composition of Jupiter's atmosphere which suggests that it accreted most of its heavy element abundance farther than tens of AU away from the Sun.

研究动机与目标

• 使用天体化学盘演化模型预测0.5–4 AU范围内温带气态巨行星的C/O与N/O比值。
• 检验木星的观测大气成分是否与通过星子吸积在内盘（5 AU以内）形成一致。
• 研究碳难熔物侵蚀前沿位置变化对行星大气化学的影响。
• 评估盘金属度对观测到的质量-金属度关系与C/O关系离散度的影响。
• 探讨未来望远镜如JWST与ARIEL对温带木星进行化学表征的可行性。

提出的方法

• 使用星子形成的人口合成模型生成半长轴在0.5–4 AU之间的合成温带木星。
• 模拟盘气体、冰与难熔物组分的天体化学演化，以计算随时间变化的C/O与N/O比值。
• 追踪固态物质（冰与难熔物）向原行星的吸积，整合金属度相关的盘特性。
• 应用两种碳难熔物侵蚀模型，评估C/O比值对碳侵蚀前沿位置的敏感性。
• 将预测的C/O与N/O比值与木星及其他系外行星（包括WASP-167e）的观测值进行比较。
• 采用固定的行星大气质量截断值以确定难熔物向行星大气的输送，尽管承认在模拟深层大气混合方面存在局限性。

实验结果

研究问题

• RQ1 通过内盘（5 AU以内）的星子吸积能否再现木星的观测C/O与N/O比值？
• RQ2 高金属度盘（[Fe/H] > 0.1）如何影响观测到的质量-金属度关系与C/O关系的离散度？
• RQ3 移动碳难熔物侵蚀前沿对气态巨行星C/O比值有何影响？
• RQ4 为何温带木星对难熔物吸积的敏感性与海王星级行星不同？
• RQ5 在轨道与观测约束下，哪些系外行星最适于未来通过JWST或ARIEL进行化学表征？

主要发现

• 合成的温带木星再现了固态吸积质量与大气C/O比值之间的观测主序列，验证了模型框架的有效性。
• 木星的大气C/O与N/O比值与在5 AU以内形成的预测结果不一致，表明其很可能形成于数十AU之外。
• 高金属度盘（[Fe/H] > 0.1）导致C/O与N/O比值的离散度更大，这可能解释了质量-金属度关系中的观测离散度。
• 移动碳侵蚀前沿显著改变木星与海王星级行星的C/O比值，但对温带土星的影响较小，因其碳与氧吸积趋于平衡。
• 引入N/O比值进一步强化了木星未在内盘形成的结论，与Öberg & Wordsworth（2019）及Bosman等（2019）的独立化学分析一致。
• 仅在碳侵蚀前沿之外吸积大气的行星极为罕见，表明大多数气态巨行星均经历了难熔物与气态物质的混合贡献。