[论文解读] Dust in brown dwarfs and extra-solar planets II. Cloud formation for cosmologically evolving abundances

S. Witte, Ch. Helling+1 more|arXiv (Cornell University)|Aug 25, 2009

Astrophysics and Star Formation Studies参考文献 61被引用 4

一句话总结

本研究利用Drift-Phoenix代码，对宇宙金属度演化过程中棕矮星和系外行星中的尘埃云形成进行建模，发现即使在[M/H] = -6.0时尘埃云依然存在。关键发现是尘埃消光系数和颗粒组成随金属度非线性演化，低金属度下颗粒尺寸减小，Fe/MgO主导性增强，尽管观测特征微弱，但对大气结构和光谱外观有显著影响。

ABSTRACT

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.

研究动机与目标

• 理解亚恒星大气中尘埃云形成如何依赖于宇宙学演化金属度，特别是对古老、贫金属天体的影响。
• 研究尘埃形成对大气结构、消光系数及气相元素耗竭的反馈作用。
• 确定在典型早期宇宙亚恒星天体中常见的极端贫金属条件下，尘埃云是否能够形成并持续存在。
• 评估由于强尘埃诱导耗竭，基于光谱线的元素丰度测定结果的可靠性。

提出的方法

• 使用矩方法方程对准静态平衡下尘埃颗粒的成核、动力学生长、蒸发及重力沉降进行数值模拟。
• 与Phoenix大气模型集成，计算一致的(T, p, v_conv)结构及合成光谱。
• 引入元素守恒方程并结合对流超射补充机制，以模拟气相元素耗竭。
• 在[M/H] ∈ [+0.5, -6.0]范围内进行网格化建模，系统研究金属度对云结构的影响。
• 采用改进的方程态（EOS）与数值算法，提升模型精度与光谱拟合能力。
• 计算尘气比、平均颗粒尺寸及颗粒组成随深度与金属度的变化关系。

实验结果

研究问题

• RQ1 在极低金属度（如[M/H] = -6.0）下，尘埃云是否能在亚恒星大气中形成并保持稳定？
• RQ2 尘埃消光系数与颗粒组成如何随金属度降低而演化？其变化是否与[M/H]呈线性关系？
• RQ3 尘埃形成对大气温度结构有何影响，特别是对温度逆增现象的形成机制？
• RQ4 尘埃诱导的气相元素耗竭在多大程度上影响基于光谱线的元素丰度测定准确性？
• RQ5 表面重力与有效温度在多大程度上调节亚恒星天体抵抗尘埃云形成的能力？

主要发现

• 尘埃云即使在[M/H] = -6.0时仍能形成并保持稳定，表明在最贫金属的亚恒星大气中尘埃形成并未被抑制。
• 尘气比不随金属度线性变化；由于云层向内迁移导致局部气体密度升高，其下降速度慢于预期。
• 平均颗粒尺寸随金属度降低而减小，导致低金属度下气相元素耗竭效率降低。
• 颗粒组成从太阳金属度附近的Mg/Si-硅酸盐主导，转变为极端贫金属天体云层底部以Fe[s]-主导，尤其在高表面重力条件下更为显著。
• 在低金属度下形成温度逆增，显著改变云结构，并抑制上层云层中硅酸盐的主导性。
• 尽管光谱特征微弱，尘埃云仍通过其消光系数与空间分布，显著影响近红外与中红外光谱，即使在[M/H] < -2.5以下亦然。