No Umbrella Needed: Confronting the Hypothesis of Iron Rain on WASP-76b with Post-processed General Circulation Models
No Umbrella Needed: Confronting the Hypothesis of Iron Rain on WASP-76b with Post-processed General Circulation Models
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不需要伞：用后处理的大气环流模型对抗 WASP-76b 上的铁雨假设
DOI：
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发表时间：
2021
期刊：
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通讯作者：
E. Rauscher
中科院分区：
文献类型：
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作者：
A. Savel;E. Kempton;M. Malik;T. Komacek;J. Bean;E. May;K. Stevenson;M. Mansfield;E. Rauscher
高分辨率光谱是系外行星大气中三维(3D)过程的独特指示器。例如，在2020年，Ehrenreich等人。据报道，来自浓缩咖啡光谱仪的透射谱从超热的木星WASP-76b产生了异常大的多普勒蓝移。对这些观测的解释让人联想到地球大气层中温度较低区域的气相铁凝结的玩具模型。在这项工作中，我们使用双灰色大气环流模式(GCMS)和射线辐射传输对WASP-76b的大气进行了正演模拟，以诊断该行星的高分辨率透射谱。我们确认，必须存在一种物理机制来驱动穿过终结点的强烈的东西不对称，才能在WASP-76b的传输光谱中再现大的多普勒蓝移。我们确定低大气阻力和深辐射-对流边界是我们的GCM产生这种不对称性的必要成分(后者与现有的斯皮策相曲线一致)。然而，我们不能仅用气相铁凝聚来再现WASP-76b多普勒信号的大小或随时间的变化。相反，我们发现由Al_2O_3、Fe或Mg_2SiO_4组成的高海拔、光学厚度很厚的云为Ehrenreich等人提供了合理的拟合。观测值--凝结的边际贡献。通过允许较小的轨道偏心率(e≈0.017)，与先前的wasp-76b轨道约束一致，进一步改善了这种适配性。我们还通过再现Tabernero等人在WASP-76b中检测到的几乎所有物种的线来验证我们的正向模拟光谱。我们的程序在诊断相分辨多普勒频移方面的成功证明了物理的、自洽的、3D模拟在模拟系外行星大气的高分辨率光谱方面的好处。
High-resolution spectra are unique indicators of three-dimensional (3D) processes in exoplanetary atmospheres. For instance, in 2020, Ehrenreich et al. reported transmission spectra from the ESPRESSO spectrograph yielding an anomalously large Doppler blueshift from the ultrahot Jupiter WASP-76b. Interpretations of these observations invoke toy model depictions of gas-phase iron condensation in lower-temperature regions of the planet’s atmosphere. In this work, we forward model the atmosphere of WASP-76b with double-gray general circulation models (GCMs) and ray-striking radiative transfer to diagnose the planet’s high-resolution transmission spectrum. We confirm that a physical mechanism driving strong east–west asymmetries across the terminator must exist to reproduce large Doppler blueshifts in WASP-76b’s transmission spectrum. We identify low atmospheric drag and a deep radiative-convective boundary as necessary components of our GCM to produce this asymmetry (the latter is consistent with existing Spitzer phase curves). However, we cannot reproduce either the magnitude or the time-dependence of the WASP-76b Doppler signature with gas-phase iron condensation alone. Instead, we find that high-altitude, optically thick clouds composed of Al2O3, Fe, or Mg2SiO4 provide reasonable fits to the Ehrenreich et al. observations—with marginal contributions from condensation. This fit is further improved by allowing a small orbital eccentricity (e ≈ 0.017), consistent with prior WASP-76b orbital constraints. We additionally validate our forward-modeled spectra by reproducing lines of nearly all species detected in WASP-76b by Tabernero et al. Our procedure’s success in diagnosing phase-resolved Doppler shifts demonstrates the benefits of physical, self-consistent, 3D simulations in modeling high-resolution spectra of exoplanet atmospheres.