Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b I. Cloud and chemistry mapping
Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b I. Cloud and chemistry mapping
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了解超热木星 HAT-P-7b 的大气特性和化学成分 I. 云和化学测绘
DOI：
10.1051/0004-6361/201935771
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发表时间：
2019
影响因子：
6.5
通讯作者：
Helling C
中科院分区：
文献类型：
--
作者：
Helling C
目前已知的约 3900 颗系外行星中，有约 100 颗可进行稀疏光谱观测。超热木星最近引起了观察者和理论家的兴趣，因为它们提供了可观察到的测试用例。将详细的理论模型与观测结果进行对比对于为即将到来的天基望远镜做好准备至关重要。目标我们的目标是研究超热木星 HAT-P-7b 上的云形成、由此产生的局部气相成分，以及它们的整体变化如何影响用于推导行星基本特性的波长相关观测结果。方法我们应用分层建模方法作为虚拟实验室来研究云形成和气相化学。我们利用 HAT-P-7b 3D GCM 的 97 个垂直 1D 剖面来评估我们的动力学云形成模型与局部平衡气相成分的一致性。我们使用地图和切片视图来提供对云和气体化学的全局了解。结果 HAT-P-7b 上的昼夜温差 (ΔT≈ 2500 K) 导致云在夜间（以 H2/He 为主）形成，而白天（以 H/He 为主）保留无云的赤道区域。云粒子的成分和大小在云的整个垂直延伸范围内变化，而且在全球范围内也变化。半径为 cm 大小的 TiO2[s]/Al2O3[s]/CaTiO3[s] 颗粒出现在较高的朝阳面纬度，导致朝阳面以气相不透明为主。然而，背面的不透明度主要由 0.01…0.1μm 的颗粒组成，这些颗粒由以硅酸盐为主的材料混合物制成。在多云区域，大气变得光学厚的气压约为 10−4 bar，在无云区域约为 0.1 bar。 结论 HAT-P-7b 具有强烈的晨/晚终止线不对称性，提供了斑块云和方位角不均匀化学的示例。可变的终结器属性可以通过入口/出口透射光度测定法（例如，CHEOPS 和 PLATO）或光谱法来获取。约 2500 K 的巨大温差导致从夜间到白天的几何延伸不断增加。在给定压力下，终止线处的 H2O 丰度随海拔变化 <1 dex，并且在给定压力下，终止线处的 H2O 丰度变化为 ≲0.3 dex（系数 2），这表明从透射光谱得出的 H2O 丰度可以代表 P≳ 10 bar 时充分混合的金属丰度。我们建议将大气 C/O 作为追踪系外行星大气中云的存在和位置的重要工具。由于云的形成，大气中的 C/O 可以是太阳下和太阳上的。 HAT-P-7b 的相位曲线变化不太可能是由白天云引起的。
ContextOf the presently known ≈3900 exoplanets, sparse spectral observations are available for ≈100. Ultra-hot Jupiters have recently attracted interest from observers and theoreticians alike, as they provide observationally accessible test cases. Confronting detailed theoretical models with observations is of preeminent importance in preparation for upcoming space-based telescopes.AimsWe aim to study cloud formation on the ultra-hot Jupiter HAT-P-7b, the resulting composition of the local gas phase, and how their global changes affect wavelength-dependent observations utilised to derive fundamental properties of the planet.MethodsWe apply a hierarchical modelling approach as a virtual laboratory to study cloud formation and gas-phase chemistry. We utilise 97 vertical 1D profiles of a 3D GCM for HAT-P-7b to evaluate our kinetic cloud formation model consistently with the local equilibrium gas-phase composition. We use maps and slice views to provide a global understanding of the cloud and gas chemistry.ResultsThe day/night temperature difference on HAT-P-7b (ΔT≈ 2500 K) causes clouds to form on the nightside (dominated by H2/He) while the dayside (dominated by H/He) retains cloud-free equatorial regions. The cloud particles vary in composition and size throughout the vertical extension of the cloud, but also globally. TiO2[s]/Al2O3[s]/CaTiO3[s]-particles of cm-sized radii occur in the higher dayside-latitudes, resulting in a dayside dominated by gas-phase opacity. The opacity on the nightside, however, is dominated by 0.01…0.1μm particles made of a material mix dominated by silicates. The gas pressure at which the atmosphere becomes optically thick is ~10−4bar in cloudy regions, and ~0.1 bar in cloud-free regions.ConclusionsHAT-P-7b features strong morning/evening terminator asymmetries, providing an example of patchy clouds and azimuthally-inhomogeneous chemistry. Variable terminator properties may be accessible by ingress/egress transmission photometry (e.g., CHEOPS and PLATO) or spectroscopy. The large temperature differences of ≈2500 K result in an increasing geometrical extension from the night- to the dayside. The H2O abundance at the terminator changes by <1 dex with altitude and ≲0.3 dex (a factor of 2) across the terminator for a given pressure, indicating that H2O abundances derived from transmission spectra can be representative of the well-mixed metallicity atP≳ 10 bar. We suggest the atmospheric C/O as an important tool to trace the presence and location of clouds in exoplanet atmospheres. The atmospheric C/O can be sub- and supersolar due to cloud formation. Phase curve variability of HAT-P-7b is unlikely to be caused by dayside clouds.