[论文解读] A new chemical scheme to study carbon-rich exoplanet atmospheres

Olivia Vénot, Éric Hébrard+3 more|arXiv (Cornell University)|Feb 12, 2015

Atmospheric Ozone and Climate被引用 1

一句话总结

本文提出了一种用于模拟富碳系外行星大气的新型C0–C6化学机制，该机制在多种温度和压力条件下均得到验证。结果表明，在C/O > 1的大气中，尤其是上层大气中，完整的C6以内烃类网络对准确的光化学建模至关重要；尽管如此，合成光谱在机制选择下仍基本保持不变。

ABSTRACT

Atmospheres with a high C/O ratio are expected to contain an important quantity of hydrocarbons, including heavy molecules (with more than 2 carbon atoms). To study correctly these C-rich atmospheres, a chemical scheme adapted to this composition is necessary. We have implemented a chemical scheme that can describe the kinetics of species with up to 6 carbon atoms. This chemical scheme has been developed with specialists of combustion and validated through experiments on a wide range of T and P. This chemical network is available on the online database KIDA. We have created a grid of 12 models to explore different thermal profiles and C/O ratios. For each of them, we have compared the chemical composition determined with a C0-C2 chemical scheme (species with up to 2 carbon atoms) and with the C0-C6 scheme. We found no difference in the results obtained with the two schemes when photolyses are not included in the model, whatever the temperature of the atmosphere. In contrast, when there is photochemistry, differences can appear in the upper atmosphere. These differences are found for all the tested PT profiles in the case that the C/O ratio is above 1. When the C/O ratio of the atmosphere is solar, differences are only found at temperatures lower than 1000K. The differences linked to the use of different chemical schemes do not have important influence on the synthetic spectra. However, we have confirmed that C2H2 and HCN as possible tracers of warm C-rich atmospheres. The use of this new chemical scheme is mandatory to model atmospheres with a high C/O ratio and, in particular, if one is interested in studying in details the photochemistry. If one is just interested in the synthetic spectra, the use of a smaller scheme may be sufficient.

研究动机与目标

• 开发一种能够模拟富碳系外行星大气中最多含六个碳原子的烃类的综合性化学机制。
• 利用实验数据和燃烧化学专业知识，在广泛的温度和压力条件下验证该机制。
• 评估在富碳系外行星大气中引入更高阶烃类对大气成分和合成光谱的影响。
• 确定在何种条件下C0–C2机制不足以替代C0–C6机制，特别是在光化学条件下。

提出的方法

• 基于燃烧化学专家的指导，构建包含最多六个碳原子物种的详细化学网络。
• 利用广泛的温度和压力范围内的实验数据对网络进行验证。
• 在12组具有不同热结构和C/O比的气象模型中实现C0–C6机制。
• 在相同条件下（包括和不包括光解离）对比C0–C2与C0–C6机制所得的化学成分。
• 生成合成光谱以评估化学机制选择对可观测特征的影响。
• 使用KIDA数据库以确保化学网络的公开可用性和可重复性。

实验结果

研究问题

• RQ1 在高C/O比系外行星大气中，包含至C6的烃类如何影响预测的化学成分？
• RQ2 在何种大气条件下，C0–C2与C0–C6化学机制之间的差异变得显著？
• RQ3 光化学如何影响简化与扩展化学网络结果之间的差异？
• RQ4 化学机制的选择在多大程度上影响富碳系外行星的合成光谱？
• RQ5 在不同C/O比下，哪些分子可作为温暖富碳大气的可靠示踪剂？

主要发现

• 当不考虑光解离时，无论大气温度如何，C0–C2与C0–C6机制之间在化学成分上均无显著差异。
• 在所有测试的热结构下，当C/O比超过1时，上层大气中出现了光化学差异。
• 在太阳C/O比条件下，仅当温度低于1000 K时，两种机制之间才出现差异。
• 合成光谱并未因化学机制选择而显著改变，表明在光谱建模中使用更简单的机制可能已足够。
• C2H2和HCN被证实是温暖富碳大气的稳健示踪剂。
• C0–C6化学机制对于高C/O比系外行星大气中光化学的准确建模至关重要。