[论文解读] Angular momentum redistribution by mixed modes in evolved low-mass stars. II. Spin-down of the core of red giants induced by mixed modes

K. Belkacem, J. P. Marques+7 more|arXiv (Cornell University)|May 20, 2015

Stellar, planetary, and galactic studies参考文献 48被引用 38

一句话总结

本文利用CoRoT和Kepler的类地观测数据，研究演化低质量恒星中混合模态对角动量的输运。研究发现，混合模态在演化红巨星的氢燃烧壳层中能高效地从核心提取角动量，从而抵消核心因收缩导致的自转加快——这为观测到的核心自转减慢提供了合理解释，但在亚巨星和早期红巨星阶段则不足以解释该现象。

ABSTRACT

The detection of mixed modes in subgiants and red giants by the CoRoT and \emph{Kepler} space-borne missions allows us to investigate the internal structure of evolved low-mass stars. In particular, the measurement of the mean core rotation rate as a function of the evolution places stringent constraints on the physical mechanisms responsible for the angular momentum redistribution in stars. It showed that the current stellar evolution codes including the modelling of rotation fail to reproduce the observations. An additional physical process that efficiently extracts angular momentum from the core is thus necessary. Our aim is to assess the ability of mixed modes to do this. To this end, we developed a formalism that provides a modelling of the wave fluxes in both the mean angular momentum and the mean energy equations in a companion paper. In this article, mode amplitudes are modelled based on recent asteroseismic observations, and a quantitative estimate of the angular momentum transfer is obtained. This is performed for a benchmark model of 1.3 $M_{\odot}$ at three evolutionary stages, representative of the evolved pulsating stars observed by CoRoT and Kepler. We show that mixed modes extract angular momentum from the innermost regions of subgiants and red giants. However, this transport of angular momentum from the core is unlikely to counterbalance the effect of the core contraction in subgiants and early red giants. In contrast, for more evolved red giants, mixed modes are found efficient enough to balance and exceed the effect of the core contraction, in particular in the hydrogen-burning shell. Our results thus indicate that mixed modes are a promising candidate to explain the observed spin-down of the core of evolved red giants, but that an other mechanism is to be invoked for subgiants and early red giants.

研究动机与目标

• 评估混合模态在提取演化低质量恒星核心角动量方面的效率。
• 解决观测到的红巨星核心自转减慢与标准恒星演化模型预测之间的差异。
• 利用CoRoT和Kepler任务观测到的模态振幅量化角动量输运。
• 确定混合模态是否足以解释演化红巨星中观测到的缓慢核心自转。
• 识别混合模态在角动量再分布中变得有效的演化阶段。

提出的方法

• 将论文I中的变换欧拉平均（TEM）形式化方法改编，用于建模平均角动量和能量方程中的波通量。
• 采用渐近近似、准绝热近似和慢旋转近似，描述致密辐射区中的波场。
• 基于CoRoT和Kepler观测到的频率间距和旋转分裂，建模混合模态振幅。
• 使用平均角动量方程中的波动量通量项（公式18）计算角动量通量。
• 将混合模态输运角动量的时间尺度与恒星收缩时间尺度（公式19）进行比较。
• 在三个演化阶段评估输运效率：亚巨星（M0）、早期红巨星（M1）和演化红巨星（M2）。

实验结果

研究问题

• RQ1 混合模态是否能从演化低质量恒星核心提取足够角动量，以解释观测到的核心自转减慢？
• RQ2 在哪个演化阶段，混合模态从低效转变为高效的角动量输运？
• RQ3 混合模态输运角动量的效率如何随半径和演化阶段变化？
• RQ4 混合模态提取的角动量是否足够强，以抵消核心收缩引起的自转加快？
• RQ5 在其他输运机制（如经向环流或内波）的背景下，混合模态起什么作用？

主要发现

• 混合模态从亚巨星和红巨星的最内层区域提取角动量，尤其在演化红巨星的氢燃烧壳层中最为显著。
• 对于亚巨星（M0）和早期红巨星（M1）模型，混合模态提取的角动量与核心收缩相比可忽略不计。
• 在演化红巨星模型（M2）中，混合模态输运的角动量足以抵消核心收缩，从而实现核心自转减慢。
• 混合模态角动量输运效率的最大值出现在浮力频率达到峰值的氢燃烧壳层。
• 尽管波振幅较高，混合模态在恒星极中心区域的效率反而降低。
• 结果表明，混合模态是演化红巨星核心自转减慢的可行机制，但无法单独解释亚巨星和早期红巨星的自转减慢。