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arXiv 2404.02604astro-ph.EP
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富水系外行星大气的非局部热动平衡模拟:冷却率与加热率

NLTE modelling of water-rich exoplanet atmospheres. Cooling and heating rates

A. García Muñoz, A. Asensio Ramos, A. Faure

AI总结:

本文通过建立H2O非局部热动平衡模型,研究富水亚海王星大气中H2O的冷却与加热机制,发现振动NLTE冷却可显著增强并部分抵消恒星加热,主张在质量损失率计算中必须考虑NLTE效应。

摘要:

氢分子和水分子对行星大气中发生的碰撞-辐射过程响应截然不同。自然产生的问题是:如果富H2O大气能够更(或更不)有效地辐射掉入射的恒星能量,那么它们对长期质量损失的抵抗能力是否比以H2为主的更强(或更弱)。如果得到证实,这一发现将影响我们对亚海王星系外行星演化的理解。作为回答这一问题的关键一步,我们提出了一个针对气体加速逃逸行星的大气区域、且条件与近距离亚海王星相关的H2O非局部热动平衡(NLTE)模型。我们对由H2、H2O和电子组成的等温气体进行的探索性计算表明:1)在恒星极紫外(XUV)光子通常沉积于大气的约1e-2至1e-4 dyn cm-2压力区域内,H2O处于转动LTE但振动NLTE状态。高H2O丰度和分数电离促进了振动LTE,我们报告了LTE-NLTE转变的临界密度;2)振动冷却可能局部主导转动冷却,部分原因是振转谱线的相对不透明度较小;3)即使较低的H2O丰度也能显著增强冷却,预计可抵消部分恒星加热;4)在压力大于等于0.1 dyn cm-2时,恒星红外(IR)光子沉积导致的加热是显著的。我们估计了H2O激发对气体内能的贡献,并推测了来自激发振动态的光解离。最终,我们的发现促使在富H2O大气的质量损失率计算中考虑NLTE。

英文摘要:

The hydrogen and water molecules respond very differently to the collisional-radiative processes taking place in planetary atmospheres. Naturally, the question arises whether H2O-rich atmospheres are more (or less) resilient to long-term mass loss than H2-dominated ones if they radiate away the incident stellar energy more (or less) efficiently. If confirmed, the finding would have implications on our understanding of the evolution of sub-Neptune exoplanets. As a key step towards answering this question, we present a non-local thermodynamic equilibrium (NLTE) model of H2O for the atmospheric region where the gas accelerates to escape the planet and conditions relevant to close-in sub-Neptunes. Our exploratory calculations for isothermal gas composed of H2, H2O and e- reveal that: 1) In the pressure region ~1e-2 - 1e-4 dyn cm-2 where the stellar extreme-ultraviolet (XUV) photons are typically deposited in the atmosphere, H2O is in rotational LTE but vibrational NLTE. Vibrational LTE is facilitated by high H2O abundances and fractional ionizations, and we report critical densities for the LTE-NLTE transition; 2) Vibrational cooling may locally dominate over rotational cooling, partly because of the comparatively small opacities of ro-vibrational lines; 3) Even low H2O abundances notably enhance the cooling, foreseeably offsetting some of the stellar heating; 4) Heating due to the deposition of stellar infrared (IR) photons is significant at pressures >=0.1 dyn cm-2. We estimate the contribution of H2O excitation to the internal energy of the gas and speculate on the photodissociation from the excited vibrational states. Ultimately, our findings motivate the consideration of NLTE in the mass loss rate calculations of H2O-rich atmospheres.

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