标题： 褐矮星和太阳系外行星中的尘埃 II. 宇宙丰度演化下的云形成 显示英文标题

标题： Dust in brown dwarfs and extra-solar planets II. Cloud formation for cosmologically evolving abundances

摘要： 次恒星天体拥有极其漫长的寿命。 对于较老的天体，宇宙学上的后果是重元素丰度较低，这导致了天体在金属丰度（从而在年龄）上分布广泛。 在其冷却的大气层中，尘埃云成为主导特征，影响着重元素的吸收率和剩余气相丰度。 我们研究了恒星金属丰度对次恒星大气中尘埃形成的影响，以及尘埃云结构及其对大气反馈的作用。 我们利用数值模拟，通过求解一组矩方程来确定准静态的尘埃云结构（DRIFT）。 这些方程模拟了成核过程、复合颗粒的动力学增长、它们的蒸发以及引力沉降，将其视为一个稳态的尘埃形成过程。 元素守恒方程扩展了这个方程组，包括对流超射带来的元素补充。 与大气代码（PHOENIX）集成后，可以确定一致的（T, p, v_conv）结构，并因此计算合成光谱。 我们为广泛的金属丰度范围计算了一组DRIFT-PHOENIX模型大气，以系统地研究整个宇宙演化过程中大气云结构的变化。 我们发现在褐矮星中最贫金属的大气层（[M/H]=-6.0）中也存在尘埃云。 只有最年轻的褐矮星和巨型气体行星中质量最大的天体能够抵抗尘埃形成。 对于非常低的重元素丰度，会出现温度反转，这对尘埃云结构有严重影响。 我们进一步表明，对于给定的有效温度，尘埃-气体比并不随天体的[M/H]线性变化。 显示英文摘要

摘要： Substellar objects have extremely long life-spans. The cosmological consequence for older objects are low abundances of heavy elements, which results in a wide distribution of objects over metallicity, hence over age. Within their cool atmosphere, dust clouds become a dominant feature, affecting the opacity and the remaining gas phase abundance of heavy elements. We investigate the influence of the stellar metallicity on the dust formation in substellar atmospheres and on the dust cloud structure and its feedback on the atmosphere. We utilize numerical simulations in which we solve a set of moment equations in order to determine the quasi-static dust cloud structure (DRIFT). These equations model the nucleation, the kinetic growth of composite particles, their evaporation and the gravitational settling as a stationary dust formation process. Element conservation equations augment this system of equations including the element replenishment by convective overshooting. The integration with an atmosphere code (PHOENIX) allows to determine a consistent (T, p, v_conv)-structure, and, hence, also to calculate synthetic spectra. A grid of DRIFT-PHOENIX model atmospheres was calculated for a wide range of metallicity to allow for a systematic study of atmospheric cloud structures throughout the evolution of the universe. We find dust clouds in even the most metal-poor ([M/H]=-6.0) atmosphere of brown dwarfs. Only the most massive among the youngest brown dwarfs and giant gas planets can resist dust formation. For very low heavy element abundances, a temperature inversion develops which has a drastic impact on the dust cloud structure. We further show that the dust-to-gas ratio does not scale linearly with the object's [M/H] for a given effective temperature.