[论文解读] The effect of internal gravity waves on cloud evolution in sub-stellar atmospheres

Parent, Amy, Falconer, Ruth E.+3 more|arXiv (Cornell University)|Feb 24, 2020

Astrophysics and Star Formation Studies被引用 2

一句话总结

本文研究了亚恒星大气中内部重力波如何增强尘埃成核与包覆层生长，使成核速率最高提升20倍，生长速率最高提升1.6倍。本文提出一种方法，可通过观测由这些波引起的带状云结构的光度变化来推断大气密度。

ABSTRACT

Sub-stellar objects exhibit photometric variability, which is believed to be caused by a number of processes, such as magnetically-driven spots or inhomogeneous cloud coverage. Recent models have shown that turbulent flows and waves, including internal gravity waves, may play an important role in cloud evolution. The aim of this paper is to investigate the effect of IGW on dust nucleation and dust growth, and whether observations of the resulting cloud structures could be used to recover atmospheric density information. For a simplified atmosphere in two dimensions, we numerically solved the governing fluid equations to simulate the effect on dust nucleation and mantle growth as a result of the passage of an IGW. Furthermore, we derived an expression that relates the properties of the wave-induced cloud structures to observable parameters in order to deduce the atmospheric density. Numerical simulations show that the $ ho, p, T$ variations caused by gravity waves lead to an increase of the nucleation rate by up to a factor 20, and an increase of the mantle growth rate by up to a factor 1.6, compared to their equilibrium values. An exploration of the wider parameter space shows that in absolute terms, the increase in nucleation due to IGW is stronger in cooler (T dwarfs) and TiO2-rich sub-stellar atmospheres. The relative increase, however, is greater in warmer (L dwarf) and TiO2-poor atmospheres due to conditions less suited for efficient nucleation at equilibrium. These variations lead to banded areas in which dust formation is much more pronounced, similar to the cloud structures observed on Earth. We show that IGW in the atmosphere of sub-stellar objects can produce banded clouds structures similar to that observed on Earth. We propose a method with which potential observations of banded clouds could be used to estimate the atmospheric density of sub-stellar objects.

研究动机与目标

• 研究内部重力波对亚恒星大气中尘埃成核与生长的影响。
• 确定波诱导的云结构是否可用于推断大气密度。
• 模拟重力波引起的周期性密度、压力与温度扰动对尘埃形成过程的响应。
• 推导一种基于观测的表达式，将云结构特征与亚恒星天体的大气密度关联起来。

提出的方法

• 通过二维流体动力学数值模拟，研究重力波传播及其对大气热力学的影响。
• 求解控制流体方程，计算由内部重力波引起的密度、压力与温度的时间依赖性变化。
• 利用波调制的热力学条件计算尘埃成核与包覆层生长速率。
• 通过光学厚度与浮力频率，推导观测通量比（S₀/S₁）、波扰动振幅与平衡大气密度ρ₀之间的关系。
• 利用浮力频率N与波空间尺度L₀，从波动力学估算初始密度扰动。
• 应用线性响应理论，将观测到的通量变化与大气密度关联，使用关键推断公式ρ₀ ≈ exp(N²L₀/g) / [L₀κgχ (ln(S₀/S₁) − L₀κdρd₁)]。

实验结果

研究问题

• RQ1 内部重力波如何影响亚恒星大气中尘埃成核与包覆层生长速率？
• RQ2 由于波诱导的热力学扰动，尘埃形成增强的幅度有多大？
• RQ3 由重力波引起的带状云结构是否可用于推断棕矮星的大气密度？
• RQ4 成核与生长的波诱导变化如何依赖于大气温度与化学成分（例如TiO₂丰度）？
• RQ5 光度变化中的哪些观测特征可用于估计大气气体密度？

主要发现

• 与平衡条件相比，内部重力波使尘埃成核速率最高提升20倍。
• 由于波诱导的扰动，尘埃包覆层生长速率最高提升1.6倍。
• 在较冷的T型矮星与TiO₂丰度较高的大气中，成核的绝对增强效应最强。
• 在较暖的L型矮星与TiO₂贫乏的大气中，成核的相对增强最大，因为基线成核效率较低。
• 波诱导的云结构表现出类似于地球波云的带状图案，暗示可观测信号的存在。
• 推导出的表达式可基于观测通量比S₀/S₁、波空间尺度L₀与变异性 timescale 估算大气密度ρ₀，典型参数值下ρ₀ ≈ 10⁻⁵ kg m⁻³。