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arXiv 1708.06976astro-ph.SRastro-ph.EP
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低质量恒星和系外行星云形成的自洽大气模拟

Self-consistent atmosphere modeling with cloud formation for low-mass stars and exoplanets

Diana Juncher, Uffe G. Jørgensen, Christiane Helling

备注：
17 pages, 14 + 3 figures
发表信息：
A&A 608, A70 (2017)
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摘要：

背景：低质量恒星和太阳系外行星具有超冷的大气层，在那里发生丰富的化学反应并形成云。太阳系外行星的光谱观测越来越多，需要自洽的模型大气模拟，以始终包括确定云形成的形成过程及其对大气的反馈。目标：用适用于低质量恒星和系外行星的模拟来补充MARCS模型大气服，为E-ELT、JWST、PLATO和其他即将建立的设施做准备。研究方法：MARCS程序计算恒星大气模型，提供辐射转移、大气结构和化学的自洽解。我们将MARCS与DRIFT相结合，DRIFT是一种动力学模型，描述了超冷大气中的云形成（种子形成，生长/蒸发，重力沉降，对流混合，元素耗尽）。结果：我们提出了一个小网格的自洽计算的大气模式$T_ \text{eff} = 2000 - 3000$ K与太阳初始丰度和$\log(g) = 4.5$。恒星和亚恒星大气中的云的形成对$T_\text{eff} < 2700$ K有重要影响，对$T_\text{eff} < 2400$ K的大气结构和光谱有重要影响。我们将模型的合成光谱与观测光谱进行了比较，发现它们很好地拟合了中晚型M矮星和早型L矮星的光谱。我们还测试DRIFT-MARCS的一个例子外行星，并证明我们的模拟再现斯皮策观测WASP-19 b相当好的$T_{\rm eff}=2600$ K，$\log(g)=3.2$和太阳丰度。我们的模型指向一个具有无云大气层的系外行星，该大气层具有大量的昼夜能量传输，并且没有温度反演。

英文摘要：

Context: Low-mass stars and extrasolar planets have ultra-cool atmospheres where a rich chemistry occurs and clouds form. The increasing amount of spectroscopic observations for extrasolar planets requires self-consistent model atmosphere simulations to consistently include the formation processes that determine cloud formation and their feedback onto the atmosphere. Aims: Complement the MARCS model atmosphere suit with simulations applicable to low-mass stars and exoplanets in preparation of E-ELT, JWST, PLATO and other upcoming facilities. Methods: The MARCS code calculates stellar atmosphere models, providing self-consistent solutions of the radiative transfer and the atmospheric structure and chemistry. We combine MARCS with DRIFT, a kinetic model that describes cloud formation in ultra-cool atmospheres (seed formation, growth/ evaporation, gravitational settling, convective mixing, element depletion). Results: We present a small grid of self-consistently calculated atmosphere models for $T_ \text{eff} = 2000 - 3000$ K with solar initial abundances and $\log(g) = 4.5$. Cloud formation in stellar and sub-stellar atmospheres appears for $T_\text{eff} < 2700$ K and has a significant effect on the structure and the spectrum of the atmosphere for $T_\text{eff} < 2400$ K. We have compared the synthetic spectra of our models with observed spectra and found that they fit the spectra of mid to late type M-dwarfs and early type L-dwarfs well. We also test DRIFT-MARCS for an example exoplanet and demonstrate that our simulations reproduce the Spitzer observations for WASP-19b rather well for $T_{\rm eff}=2600$ K, $\log(g)=3.2$ and solar abundances. Our model points at an exoplanet with a deep cloud-free atmosphere with a substantial day-night energy transport and no temperature inversion.

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