系外行星和褐矮星上的矿物雪花：微孔隙度、尺寸分布和颗粒形状的影响
Mineral snowflakes on exoplanets and brown dwarfs: Effects of micro-porosity, size distributions, and particle shape
摘要：
系外行星大气特征已成为研究系外行星形成、演化的重要工具。然而，云仍然是表征的一个关键挑战：即将到来的空间望远镜（如JWST，ARIEL）和地面高分辨率光谱仪（如CRIRES+）将产生需要详细了解云形成和云效应的数据。我们的目标是了解云颗粒的微孔性如何影响云的结构，颗粒大小和物质成分。我们研究了微孔粒子的光谱效应，粒子大小分布，和非球形云粒子。我们扩展了我们的动力学非平衡云形成模型，并使用规定的1D（T_gas-p_gas）DRIFT-PHOENIX配置文件的网格。我们应用有效介质理论和米氏理论来模拟云粒子的光谱特性与微孔隙度和派生的粒径分布。我们使用空心球的统计分布来表示非球形云粒子的影响。高度微孔的云颗粒（90%真空）具有更大的表面积，能够在大气中比致密颗粒更高的地方有效地生长。这些矿物雪花的单次散射和横截面积的增加导致云层仅在波长约为100 ${\rm μm}$时变得光学薄，而不是在约为20 ${\rm μm}$时变得光学薄。当云粒子出现局部变化的高斯尺寸分布时，也可以看到云粒子的显著增强。非球形颗粒增加了硅酸盐光谱特征的不透明度，这进一步增加了云变得光学薄的波长。JWST MIRI将根据云的光学薄波长对微孔和非球形云粒子的特征敏感。
英文摘要：
Exoplanet atmosphere characterisation has become an important tool in understanding exoplanet formation, evolution. However, clouds remain a key challenge for characterisation: upcoming space telescopes (e.g. JWST, ARIEL) and ground-based high-resolution spectrographs (e.g. CRIRES+) will produce data requiring detailed understanding of cloud formation and cloud effects. We aim to understand how the micro-porosity of cloud particles affects the cloud structure, particle size, and material composition. We examine the spectroscopic effects of micro-porous particles, the particle size distribution, and non-spherical cloud particles. We expanded our kinetic non-equilibrium cloud formation model and use a grid of prescribed 1D (T_gas-p_gas) DRIFT-PHOENIX profiles. We applied the effective medium theory and the Mie theory to model the spectroscopic properties of cloud particles with micro-porosity and a derived particle size distribution. We used a statistical distribution of hollow spheres to represent the effects of non-spherical cloud particles. Highly micro-porous cloud particles (90% vacuum) have a larger surface area, enabling efficient bulk growth higher in the atmosphere than for compact particles. Increases in single-scattering albedo and cross-sectional area for these mineral snowflakes cause the cloud deck to become optically thin only at a wavelength of ~100 ${\rm μm}$ instead of at the ~20 ${\rm μm}$ for compact cloud particles. A significant enhancement in albedo is also seen when cloud particles occur with a locally changing Gaussian size distribution. Non-spherical particles increase the opacity of silicate spectral features, which further increases the wavelength at which the clouds become optically thin. JWST MIRI will be sensitive to signatures of micro-porous and non-spherical cloud particles based on the wavelength at which clouds are optically thin.
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