[论文解读] 3D hydrodynamic simulations of massive main-sequence stars II. Convective excitation and spectra of internal gravity waves

W. T. Thompson, Falk Herwig+5 more|arXiv (Cornell University)|Mar 10, 2023

Stellar, planetary, and galactic studies被引用 5

一句话总结

本研究利用25 M⊙ 大质量主序星的三维流体动力学模拟，研究了对流激发内部重力波（IGWs）及其可观测特征。结果表明，核心对流以随机方式在广泛的角阶数范围内激发IGWs，在模拟的光变曲线中产生低频功率过剩，其特征与观测到的星震学数据定性相符，尽管模式相干性较低且寿命仅为约10–100天，IGW的可观测功率仍可延伸至包层布伦特–V€äis€äl€频率。

ABSTRACT

Recent photometric observations of massive stars have identified a low-frequency power excess which appears as stochastic low-frequency variability in light curve observations. We present the oscillation properties of high resolution hydrodynamic simulations of a 25 $\mathrm{M}_\odot$ star performed with the PPMStar code. The model star has a convective core mass of $\approx\, 12\, \mathrm{M}_\odot$ and approximately half of the envelope simulated. From this simulation, we extract light curves from several directions, average them over each hemisphere, and process them as if they were real photometric observations. We show how core convection excites waves with a similar frequency as the convective time scale in addition to significant power across a forest of low and high angular degree $l$ modes. We find that the coherence of these modes is relatively low as a result of their stochastic excitation by core convection, with lifetimes on the order of 10s of days. Thanks to the still significant power at higher $l$ and this relatively low coherence, we find that integrating over a hemisphere produces a power spectrum that still contains measurable power up to the Brunt--Väisälä frequency. These power spectra extracted from the stable envelope are qualitatively similar to observations, with same order of magnitude yet lower characteristic frequency. This work further shows the potential of long-duration, high-resolution hydrodynamic simulations for connecting asteroseismic observations to the structure and dynamics of core convection and the convective boundary.

研究动机与目标

• 通过高分辨率三维流体动力学模拟，理解大质量主序星中内部重力波（IGWs）的激发机制。
• 将核心对流对IGWs的随机激发与大质量恒星中的可观测光变变异性联系起来。
• 评估模拟光变曲线的半球积分在多大程度上再现了TESS和K2观测中发现的低频功率过剩。
• 研究核心对流在不同角阶数下激发IGWs及其相干性特征的作用。
• 探索IGW谱作为探测大质量恒星核心对流与包层结构潜力的可能性。

提出的方法

• 使用PPMstar代码对一颗25 M⊙ 主序星进行模拟，其核心为12 M⊙ 的对流区，包层为一半质量。
• 从多个视角提取光变曲线，并对每个半球进行平均，以模拟真实的光变观测。
• 计算半球平均光变曲线的谱，以分析内部重力波的功率分布。
• 模拟中将加热强度提高1000倍以加速核心对流并降低计算成本，随后将结果按10倍比例缩小以恢复物理相关性。
• 应用滑动傅里叶变换以识别时变模式，并估计其寿命和再激发 timescales。
• 以布伦特–V€äis€äl€频率为参考，评估光谱中可探测IGW功率的上限频率。

实验结果

研究问题

• RQ1 大质量主序星核心对流如何在不同角阶数下激发内部重力波？
• RQ2 模拟光变曲线的半球积分在多大程度上保留了非相干IGWs的低频功率？
• RQ3 由核心对流驱动的IGW激发的特征频率是多少？其与对流周转 timescale 的关系如何？
• RQ4 仅由核心对流激发IGWs是否足以再现大质量恒星中观测到的低频功率过剩？
• RQ5 单个IGW模式的寿命与相干性如何与核心对流动力学相关？

主要发现

• 核心对流以峰值约为几μHz的特征频率激发IGWs，对应于对流周转 timescale。
• 模拟产生半球平均光变曲线中的低频功率过剩，其形态与TESS和K2数据中观测到的结果定性相符。
• 尽管相干性较低且寿命仅为约10–100天，IGWs的可观测功率仍可延伸至包层布伦特–V€äis€äl€频率。
• 单个IGW模式表现出时变功率，其寿命和再激发 timescales 约为数十天，与观测趋势一致。
• 通过物质穿越对流楔的时间计算得到的特征对流频率，与IGW功率谱中的主峰相匹配。
• 观测功率谱的形状表明，低频尾部的截止频率可直接用作包层布伦特–V€äis€äl€频率的探测工具。