HD 209458b in new light: evidence of nitrogen chemistry, patchy clouds and sub-solar water
HD 209458b in new light: evidence of nitrogen chemistry, patchy clouds and sub-solar water
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DOI：
10.1093/mnras/stx804
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发表时间：
2017-01
影响因子：
4.8
通讯作者：
R. MacDonald;N. Madhusudhan
中科院分区：
文献类型：
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作者：
R. MacDonald;N. Madhusudhan
云/雾造成的明显遮蔽破坏了对系外行星透射光谱的解释。关于系外行星光谱中看到的弱 H$_2$O 特征是否是由于云层或固有的贫氧所致的争论非常激烈。太阳 H$_2$O 丰度的断言依赖于先验模型假设，例如化学/辐射平衡。在这项工作中，我们尝试用新的透射谱检索范例来解决这个问题。我们介绍 POSEIDON，一种包括广义非均匀云的二维大气反演算法。我们证明，这一处方可以打破云和突出分子丰度之间的重要简并性。我们将 POSEIDON 应用于目前可用的最佳传输光谱，即热木星 HD 209458b，揭示了对其昼夜终结者大气的新见解。我们在完全贝叶斯检索框架中使用前所未有的 10$^8$ 模型广泛探索参数空间，跨越从完全多云到无云大气的连续体。我们报告首次在系外行星大气中检测到氮化学物质（NH$_3$ 和/或 HCN），置信度为 3.7-7.7$\sigma$，不均匀云覆盖为 4.5-5.4$\sigma$，高空雾霾为 $>$3$\sigma$，亚太阳 H$_2$O 为 $\gtrsim$3-5$\sigma$，取决于假设的云分布。对于完全多云和无云的场景，我们分别在 3.3$\sigma$ 和 4.9$\sigma$ 处检测到 NH$_3$。对于具有最高贝叶斯证据的模型，我们将 H$_2$O 限制在 5-15 ppm（0.01-0.03$\times$ 太阳能），将 NH$_3$ 限制在 0.01-2.7 ppm，强烈表明不平衡化学并警告平衡假设。我们的结果预示着使用高精度 HST 和 JWST 光谱检索多云大气的新希望。
Interpretations of exoplanetary transmission spectra have been undermined by apparent obscuration due to clouds/hazes. Debate rages on whether weak H$_2$O features seen in exoplanet spectra are due to clouds or inherently depleted oxygen. Assertions of solar H$_2$O abundances have relied on making a priori model assumptions, e.g. chemical/radiative equilibrium. In this work, we attempt to address this problem with a new retrieval paradigm for transmission spectra. We introduce POSEIDON, a two-dimensional atmospheric retrieval algorithm including generalised inhomogeneous clouds. We demonstrate that this prescription allows one to break vital degeneracies between clouds and prominent molecular abundances. We apply POSEIDON to the best transmission spectrum presently available, for the hot Jupiter HD 209458b, uncovering new insights into its atmosphere at the day-night terminator. We extensively explore the parameter space with an unprecedented 10$^8$ models, spanning the continuum from fully cloudy to cloud-free atmospheres, in a fully Bayesian retrieval framework. We report the first detection of nitrogen chemistry (NH$_3$ and/or HCN) in an exoplanet atmosphere at 3.7-7.7$\sigma$ confidence, non-uniform cloud coverage at 4.5-5.4$\sigma$, high-altitude hazes at $>$3$\sigma$, and sub-solar H$_2$O at $\gtrsim$3-5$\sigma$, depending on the assumed cloud distribution. We detect NH$_3$ at 3.3$\sigma$ and 4.9$\sigma$ for fully cloudy and cloud-free scenarios, respectively. For the model with the highest Bayesian evidence, we constrain H$_2$O at 5-15 ppm (0.01-0.03$\times$ solar) and NH$_3$ at 0.01-2.7 ppm, strongly suggesting disequilibrium chemistry and cautioning against equilibrium assumptions. Our results herald new promise for retrieving cloudy atmospheres using high-precision HST and JWST spectra.