[论文解读] Seismic diagnostics for transport of angular momentum in stars 1. Rotational splittings from the PMS to the RGB

J. P. Marques, Goupil, M. J.+13 more|arXiv (Cornell University)|Nov 6, 2012

Stellar, planetary, and galactic studies参考文献 82被引用 140

一句话总结

本研究利用地震诊断方法，从主序前阶段到红巨星分支，研究类太阳恒星的旋转分裂，以约束角动量输运。结果发现，标准模型因输运机制不足而高估了核心旋转速度，表明水平湍流粘性可能被低估，或需要额外的物理机制来解释红巨星中观测到的缓慢核心旋转。

ABSTRACT

Rotational splittings are currently measured for several main sequence stars and a large number of red giants with the space mission Kepler. This will provide stringent constraints on rotation profiles. Our aim is to obtain seismic constraints on the internal transport and surface loss of angular momentum of oscillating solar-like stars. To this end, we study the evolution of rotational splittings from the pre-main sequence to the red-giant branch for stochastically excited oscillation modes. We modified the evolutionary code CESAM2K to take rotationally induced transport in radiative zones into account. Linear rotational splittings were computed for a sequence of $1.3 M_{\odot}$ models. Rotation profiles were derived from our evolutionary models and eigenfunctions from linear adiabatic oscillation calculations. We find that transport by meridional circulation and shear turbulence yields far too high a core rotation rate for red-giant models compared with recent seismic observations. We discuss several uncertainties in the physical description of stars that could have an impact on the rotation profiles. For instance, we find that the Goldreich-Schubert-Fricke instability does not extract enough angular momentum from the core to account for the discrepancy. In contrast, an increase of the horizontal turbulent viscosity by 2 orders of magnitude is able to significantly decrease the central rotation rate on the red-giant branch. Our results indicate that it is possible that the prescription for the horizontal turbulent viscosity largely underestimates its actual value or else a mechanism not included in current stellar models of low mass stars is needed to slow down the rotation in the radiative core of red-giant stars.

研究动机与目标

• 利用地震观测约束低质量恒星内部角动量输运与表面损失。
• 检验当前恒星模型在重现主序前至红巨星分支阶段观测到的旋转分裂方面的有效性。
• 识别红巨星中预测与观测到的核心旋转速率之间的差异。
• 评估经向环流、剪切湍流及替代输运机制在解释地震数据中的作用。

提出的方法

• 修改CESAM2K恒星演化代码，以包含辐射区中由旋转引起的输运。
• 计算从主序前到红巨星分支的1.3 M☉演化序列的线性旋转分裂。
• 从自洽的演化模型中推导旋转分布，并通过线性无量温振荡计算获得本征函数。
• 采用Mathis等人（2004）的湍流粘性公式处理水平扩散，采用Talon与Zahn（1997）的公式处理垂直扩散。
• 应用Goldreich-Schubert-Fricke不稳定性准则，评估其在角动量提取中的潜在作用。
• 探索将水平湍流粘性提高两个数量级对核心旋转的影响。

实验结果

研究问题

• RQ1 为何标准恒星模型预测的红巨星核心旋转速率远高于地震观测结果？
• RQ2 经向环流与剪切湍流在多大程度上可解释低质量恒星中观测到的旋转分裂？
• RQ3 Goldreich-Schubert-Fricke不稳定性是否足以提取足够角动量，以弥合模型与观测之间的差异？
• RQ4 将水平湍流粘性提高两个数量级，能否使预测的核心旋转速率降低至与观测一致？
• RQ5 当前一维恒星模型中可能缺少哪些物理机制，以解释红巨星中缓慢的核心旋转？

主要发现

• 仅靠经向环流与剪切湍流，红巨星模型的核心旋转速率仍高于地震观测结果。
• Goldreich-Schubert-Fricke不稳定性无法提取足够角动量以解决模型与观测之间的差异。
• 将水平湍流粘性提高两个数量级，可显著降低红巨星分支上的中心旋转速率。
• 结果表明，标准水平湍流粘性公式在恒星内部可能严重低估了其真实值。
• 为解释低质量红巨星中观测到的缓慢核心旋转，可能需要额外的、未建模的角动量输运机制。
• 地震诊断为内部旋转分布提供了强有力约束，并揭示了当前恒星演化模型的局限性。