Signatures of Nitrogen Chemistry in Hot Jupiter Atmospheres
Signatures of Nitrogen Chemistry in Hot Jupiter Atmospheres
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DOI：
10.3847/2041-8213/aa97d4
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发表时间：
2017-11
期刊：
影响因子：
--
通讯作者：
R. MacDonald;N. Madhusudhan
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文献类型：
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作者：
R. MacDonald;N. Madhusudhan
行星外大气的分子组成的推断通常集中在碳、氢和含氧分子上。最近，哈勃太空望远镜广域相机3的透射谱在1.55μm附近的额外吸收被归因于含氮的化学物种：NH3或HCN。这些物种如果大量存在，将是不平衡化学过程的强烈指标--例如，垂直混合和光化学。由此得出的氮丰度反过来也可能对行星形成机制产生重要的制约作用。在这里，我们检查了行星外大气中氮化学的可探测性。除了WFC3带通(1.1-1.7μm)外，我们还发现在∼2.2μm的K波段观测到了强的NH3特征，而在∼3.1μm和∼4.0μm的观测则采样了强的HCN特征。在对这样的观测的预期中，我们预测了典型的热木星可能在1-5μm光谱范围内由氮化学引起的吸收特征幅度。最后，我们对九个热木星的大气透射谱进行了大气反演，以寻找暗示氮化学的近红外吸收特征。我们报告了在WASP-31b(2.2σ)中检测到NH_3，在WASP-63b(2.3σ)中检测到HCN，以及在HD 209458b中可能归因于NH_3的过量吸收。从1到5μm的高精度观测(例如，使用詹姆斯·韦伯太空望远镜)将能够对氮化学进行明确的探测，进而对不平衡的大气化学和行星形成过程进行强有力的诊断。
Inferences of molecular compositions of exoplanetary atmospheres have generally focused on carbon, hydrogen, and oxygen-bearing molecules. Recently, additional absorption in Hubble Space Telescope Wide Field Camera 3 (WFC3) transmission spectra around 1.55 μm has been attributed to nitrogen-bearing chemical species: NH3 or HCN. These species, if present in significant abundance, would be strong indicators of disequilibrium chemical processes—e.g., vertical mixing and photochemistry. The derived N abundance, in turn, could also place important constraints on planetary formation mechanisms. Here, we examine the detectability of nitrogen chemistry in exoplanetary atmospheres. In addition to the WFC3 bandpass (1.1–1.7 μm), we find that observations in the K-band at ∼2.2 μm, achievable with present ground-based telescopes, sample a strong NH3 feature, while observations at ∼3.1 μm and ∼4.0 μm sample strong HCN features. In anticipation of such observations, we predict absorption feature amplitudes due to nitrogen chemistry in the 1–5 μm spectral range possible for a typical hot Jupiter. Finally, we conduct atmospheric retrievals of nine hot Jupiter transmission spectra in search of near-infrared absorption features suggestive of nitrogen chemistry. We report weak detections of NH3 in WASP-31b (2.2σ), HCN in WASP-63b (2.3σ), and excess absorption that could be attributed to NH3 in HD 209458b. High-precision observations from 1 to 5 μm (e.g., with the James Webb Space Telescope) will enable definitive detections of nitrogen chemistry, in turn serving as powerful diagnostics of disequilibrium atmospheric chemistry and planetary formation processes.