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The composition of hot Jupiter atmospheres assembled within chemically evolved protoplanetary discs
Monthly Notices of the Royal Astronomical Society ( IF 4.8 ) Pub Date : 2020-09-25 , DOI: 10.1093/mnras/staa2944
Shota Notsu 1, 2 , Christian Eistrup 2, 3 , Catherine Walsh 4 , Hideko Nomura 5
Affiliation  

The radial-dependent positions of snowlines of abundant oxygen- and carbon-bearing molecules in protoplanetary discs will result in systematic radial variations in the C/O ratios in the gas and ice. This variation is proposed as a tracer of the formation location of gas-giant planets. However, disc chemistry can affect the C/O ratios in the gas and ice, thus potentially erasing the chemical fingerprint of snowlines in gas-giant atmospheres. We calculate the molecular composition of hot Jupiter atmospheres using elemental abundances extracted from a chemical kinetics model of a disc midplane where we have varied the initial abundances and ionization rates. The models predict a wider diversity of possible atmospheres than those predicted using elemental ratios from snowlines only. As found in previous work, as the C/O ratio exceeds the solar value, the mixing ratio of CH$_{4}$ increases in the lower atmosphere, and those of C$_{2}$H$_{2}$ and HCN increase mainly in the upper atmosphere. The mixing ratio of H$_{2}$O correspondingly decreases. We find that hot Jupiters with C/O$>1$ can only form between the CO$_{2}$ and CH$_{4}$ snowlines. Moreover, they can only form in a disc which has fully inherited interstellar abundances, and where negligible chemistry has occurred. Hence, carbon-rich planets are likely rare, unless efficient transport of hydrocarbon-rich ices via pebble drift to within the CH$_{4}$ snowline is a common phenomenon. We predict combinations of C/O ratios and elemental abundances that can constrain gas-giant planet formation locations relative to snowline positions, and that can provide insight into the disc chemical history.

中文翻译:

在化学演化的原行星盘内组装的热木星大气的组成

原行星盘中丰富的含氧和含碳分子的雪线的径向相关位置将导致气体和冰中 C/O 比的系统径向变化。这种变化被提议作为气体巨行星形成位置的示踪剂。然而,圆盘化学会影响气体和冰中的 C/O 比,从而有可能消除气态巨大气中雪线的化学指纹。我们使用从圆盘中平面的化学动力学模型中提取的元素丰度来计算热木星大气的分子组成,其中我们改变了初始丰度和电离率。与仅使用雪线的元素比率预测的模型相比,这些模型预测的可能大气层的多样性更大。在之前的工作中发现,由于 C/O 比超过太阳能值,CH$_{4}$的混合比例在低层大气增加,C$_{2}$H$_{2}$和HCN的混合比例主要在高层大气增加。H$_{2}$O 的混合比例相应减小。我们发现 C/O$>1$ 的热木星只能在 CO$_{2}$ 和 CH$_{4}$ 雪线之间形成。此外,它们只能在一个完全继承星际丰度的圆盘中形成,并且发生的化学反应可以忽略不计。因此,富含碳的行星可能很少见,除非通过卵石漂移将富含碳氢化合物的冰有效运输到 CH$_{4}$ 雪线内是一种普遍现象。我们预测了 C/O 比率和元素丰度的组合,可以限制气态巨行星形成位置相对于雪线位置,并且可以提供对圆盘化学历史的深入了解。
更新日期:2020-09-25
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