Dust in brown dwarfs and extra-solar planets II. Cloud formation for cosmologically evolving abundances
Dust in brown dwarfs and extra-solar planets II. Cloud formation for cosmologically evolving abundances
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褐矮星和太阳系外行星中的尘埃 II 宇宙演化丰度的云形成
DOI：
10.1051/0004-6361/200811501
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发表时间：
2009
影响因子：
6.5
通讯作者：
Hauschildt
中科院分区：
文献类型：
--
作者：
Helling;Hauschildt
亚恒星物体的寿命非常长。年老天体的宇宙学结果是重元素的丰度很低，这反过来又导致了天体在金属丰度上的广泛分布，从而超过了年龄。在它们的冷大气中，尘埃云成为主要特征，影响着不透明度和剩余的重元素气相丰度。目的研究恒星金属丰度对亚恒星大气尘埃形成和尘埃云结构的影响及其对大气的反馈。这项工作对早期宇宙中恒星形成和尘埃形成的一般问题具有启示意义。方法利用数值模拟方法求解一组矩方程，确定准静态尘云结构（漂移）。这些方程模拟了复合颗粒的成核、动力学生长、蒸发和重力沉降作为静止尘埃形成过程。元素守恒方程通过包括对流超调的元素补充来增强这个方程组。与大气代码（Phoenix）的集成允许确定一致结构（T -局部温度，p -局部压力，v对流速度），从而计算合成光谱。结果得到了Drift-Phoenix模式大气金属丰度[M/H][+0.5, -0.0, -0.5，…][-6.0]，以便系统地研究整个宇宙演化过程中的大气云结构。我们甚至在最缺乏金属（[M/H] = -6.0）的褐矮星大气中发现尘埃云。只有最年轻的褐矮星和巨大的气体行星中质量最大的才能抵抗尘埃的形成。对于非常低的重元素丰度，会产生逆温，对尘埃云结构产生剧烈的影响。尘埃形成造成的金属耗竭和内部元素丰度的不确定性的结合，使亚恒星大气的建模成为一个复杂的问题，特别是对于古老的亚恒星天体。我们进一步表明，在给定的有效温度下，尘气比不与物体的[M/H]成线性关系。平均颗粒大小和颗粒组成随[M/H]的变化而变化，这影响了决定辐射加热和冷却的尘埃不透明度以及光谱外观。
ContextSubstellar objects have extremely long life spans. The cosmological consequence for older objects are low abundances for heavy elements, which in turn 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.AimsWe 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. This work has implications for the general questions of star formation and of dust formation in the early universe.MethodsWe utilise numerical simulations to solve a set of moment equations 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 by including the element replenishment by convective overshooting. The integration with an atmosphere code (Phoenix) allows determination of a consistent-structure (T– local temperature,p– local pressure,vconv– convective velocity), hence, to calculate synthetic spectra.ResultsA grid of Drift-Phoenix model atmospheres was calculated for a wide range of metallicity, [M/H][ +0.5, -0.0, -0.5, ..., -6.0] , to allow for 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 that has a drastic impact on the dust cloud structure.ConclusionsThe combination of metal depletion by dust formation and the uncertainty of interior element abundances makes the modelling of substellar atmospheres an intricate problem in particular for old substellar objects. We furthermore show that the dust-to-gas ratio doesnotscale linearly with the object's [M/H] for a given effective temperature. The mean grain sizes and the composition of the grains change depending on [M/H], which influences the dust opacity that determines radiative heating and cooling, as well as the spectral appearance.
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金属丰度非常低的恒星形成。
DOI：
10.1088/0004-637x/696/2/1065
发表时间：
2007
期刊：
The Astrophysical Journal
影响因子：
--
作者：
A. Jappsen;R. Klessen;S. Glover;M. Mac Low
通讯作者：
M. Mac Low
从锂到铀：早期宇宙演化的元素示踪剂，由 Vanessa Hill、Patrick François 和 Francesca Primas 编辑
影响因子：
2.2
作者：
B. Meyer
通讯作者：
B. Meyer
晚期 M 矮星表面对流的数值模拟
影响因子：
6.5
作者：
Hans;Hans;F. Allard;P. Hauschildt;P. Hauschildt
通讯作者：
P. Hauschildt
两微米全天巡天发现第二颗 L 亚矮星
DOI：
10.1086/425418
发表时间：
2004
期刊：
The Astrophysical Journal Letters
影响因子：
--
作者：
A. Burgasser
通讯作者：
A. Burgasser
恒星形成的最小质量以及双星褐矮星的起源
影响因子：
6.5
作者：
A. Whitworth;D. Stamatellos
通讯作者：
D. Stamatellos