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arXiv 1104.5477astro-ph.EP
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GJ 1214 b的大气化学:光化学和云

The Atmospheric Chemistry of GJ 1214b: Photochemistry and Clouds

Eliza Miller-Ricci Kempton, Kevin Zahnle, Jonathan J. Fortney

摘要:

最近的观测过境超级地球GJ 1214 b显示,它的大气可能是富含氢或水的性质,云或薄雾可能会影响其在光学和非常近红外的透射光谱。在这里,我们进一步研究的可能性,GJ 1214 b确实拥有一个以氢为主的大气,这是一个假设,是赞成的模型的大部分组成的行星。我们研究了非平衡化学(光化学,热化学和混合)对行星透射光谱的影响。我们还研究了云在衰减GJ 1214 b的短波长透射光谱中发挥重要作用的可能性。我们发现,非平衡化学可能对GJ 1214 b大气的整体化学成分有很大的影响,但这些变化主要发生在大气中的高度以上,这是通过透射光谱探测。因此,非平衡化学对GJ 1214 b的透射光谱的影响是最小的,如果行星的大气层具有超太阳的金属丰度和低垂直混合率,则会产生最大的影响。有趣的是,我们发现最适合GJ 1214 b大气传输观测的情况发生在行星大气缺乏CH 4,并且在约200 mbar的压力下拥有云层的情况下。这与甲烷有效光解的图像一致,伴随着有机烟雾的形成,在光学波长下掩盖了GJ 1214 b的低层大气。然而,对于GJ 1214 b的透射光谱中不存在的甲烷,该分子的UV光解必须在大于~1毫巴的压力下有效,而我们发现甲烷仅在小于0.1毫巴的压力下光解,即使在最乐观的假设下。(节录)

英文摘要:

Recent observations of the transiting super-Earth GJ 1214b reveal that its atmosphere may be hydrogen-rich or water-rich in nature, with clouds or hazes potentially affecting its transmission spectrum in the optical and very-near-IR. Here we further examine the possibility that GJ 1214b does indeed possess a hydrogen-dominated atmosphere, which is the hypothesis that is favored by models of the bulk composition of the planet. We study the effects of non-equilibrium chemistry (photochemistry, thermal chemistry, and mixing) on the planet's transmission spectrum. We furthermore examine the possibility that clouds could play a significant role in attenuating GJ 1214b's transmission spectrum at short wavelengths. We find that non-equilibrium chemistry can have a large effect on the overall chemical composition of GJ 1214b's atmosphere, however these changes mostly take place above the height in the atmosphere that is probed by transmission spectroscopy. The effects of non-equilibrium chemistry on GJ 1214b's transmission spectrum are therefore minimal, with the largest effects taking place if the planet's atmosphere has super-solar metallicity and a low rate of vertical mixing. Interestingly, we find that the best fit to the observations of GJ 1214b's atmosphere in transmission occur if the planet's atmosphere is deficient in CH4, and possesses a cloud layer at a pressure of ~200 mbar. This is consistent with a picture of efficient methane photolysis, accompanied by formation of organic haze that obscures the lower atmosphere of GJ 1214b at optical wavelengths. However, for methane to be absent from GJ 1214b's transmission spectrum, UV photolysis of this molecule must be efficient at pressures of greater than ~1 mbar, whereas we find that methane only photolyzes to pressures less than 0.1 mbar, even under the most optimistic assumptions. (Abridged)

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