Dust in brown dwarfs and extra-solar planets - I. Chemical composition and spectral appearance of quasi-static cloud layers
Dust in brown dwarfs and extra-solar planets - I. Chemical composition and spectral appearance of quasi-static cloud layers
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DOI：
10.1051/0004-6361:20078220
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发表时间：
2008-07-01
影响因子：
6.5
通讯作者：
Thi, W. -F.
中科院分区：
文献类型：
--
作者：
Helling, Ch.;Woitke, P.;Thi, W. -F.
目标。褐矮星被尘埃云层覆盖，导致表面特征不均匀，并在物体演化过程中移动到可观测的tau = 1水平以下。云层对褐矮星和太阳系外行星的结构和光谱外观有很强的影响，例如，通过提供较高的局部不透明度和通过从大气中除去可冷凝元素，导致大气中的金属丰度低于太阳。我们的目标是了解在准静态的亚恒星大气中云层的形成，这些大气是由许多不同固体凝结物的小岛屿组成的脏颗粒组成的。随时间变化的描述是描述成核、生长和蒸发的动力学模型。将其推广到处理重力沉降，并应用于亚恒星模式大气的静定情况。由尘埃矩解近似确定了粒径分布函数，并结合计算得到的物料体积分数，为应用有效介质理论和米氏理论计算复合粉尘颗粒的浊度提供了依据。褐矮星和热巨气行星中的云粒子在高层大气中被发现很小（大约0.01 μ m），并且由所有被考虑的冷凝物的丰富混合物组成，特别是MgSiO(3)[s], Mg(2)SiO(4)[s]和SiO(2)[s]。随着颗粒向下沉降，它们的尺寸增大，并在最深处达到100 μ m。晶粒中挥发性较强的部分蒸发，颗粒逐步净化，在较深层形成高温冷凝物的复合颗粒，主要由Fe[s]和Al(2)O(3)[s]组成。尘埃形成过程中所涉及的元素的气相丰度在整个大气中以数量级变化。在高层大气中，粒度分布相对较宽，而在较深层则达到峰值。这反映了在中间高度处成核过程的停止。在中红外波段（7-20 μ m），云层的光谱外观接近灰色体，只有几个百分点的微弱宽特征，主要是由MgSiO(3)[s]和Mg(2)SiO(4)[s]引起的。然而，这些特征是可以通过观测探测到的高层大气中尘埃的一个指纹。我们的模型预测，在高层大气中，气相耗竭比相平衡计算弱得多。由于密度低，那里的尘埃形成过程不完整，这导致了相当数量的剩余元素，这些元素可能会产生更强、更宽的中性金属线。
Aims. Brown dwarfs are covered by dust cloud layers which cause inhomogeneous surface features and move below the observable tau = 1 level during the object's evolution. The cloud layers have a strong influence on the structure and spectral appearance of brown dwarfs and extra-solar planets, e. g. by providing high local opacities and by removing condensable elements from the atmosphere causing a sub-solar metalicity in the atmosphere. We aim at understanding the formation of cloud layers in quasi-static substellar atmospheres that consist of dirty grains composed of numerous small islands of different solid condensates.Methods. The time-dependent description is a kinetic model describing nucleation, growth and evaporation. It is extended to treat gravitational settling and is applied to the static-stationary case of substellar model atmospheres. From the solution of the dust moments, we determine the grain size distribution function approximately which, together with the calculated material volume fractions, provides the basis for applying effective medium theory and Mie theory to calculate the opacities of the composite dust grains.Results. The cloud particles in brown dwarfs and hot giant-gas planets are found to be small in the high atmospheric layers (a approximate to 0.01 mu m), and are composed of a rich mixture of all considered condensates, in particular MgSiO(3)[s], Mg(2)SiO(4)[s] and SiO(2)[s]. As the particles settle downward, they increase in size and reach several 100 mu m in the deepest layers. The more volatile parts of the grains evaporate and the particles stepwise purify to form composite particles of high-temperature condensates in the deeper layers, mainly made of Fe[s] and Al(2)O(3)[s]. The gas phase abundances of the elements involved in the dust formation process vary by orders of magnitudes throughout the atmosphere. The grain size distribution is found to be relatively broad in the upper atmospheric layers but strongly peaked in the deeper layers. This reflects the cessation of the nucleation process at intermediate heights. The spectral appearance of the cloud layers in the mid IR (7-20 mu m) is close to a grey body with only weak broad features of a few percent, mainly caused by MgSiO(3)[s], and Mg(2)SiO(4)[s]. These features are, nevertheless, a fingerprint of the dust in the higher atmospheric layers that can be probed by observations.Conclusions. Our models predict that the gas phase depletion is much weaker than phase-equilibrium calculations in the high atmospheric layers. Because of the low densities, the dust formation process is incomplete there, which results in considerable amounts of left-over elements that might produce stronger and broader neutral metallic lines.