[论文解读] Asteroseismology of evolved stars to constrain the internal transport of angular momentum II. Test of a revised prescription for transport by the Tayler instability

P. Eggenberger, J. W. den Hartogh+4 more|arXiv (Cornell University)|Jan 30, 2020

Stellar, planetary, and galactic studies参考文献 34被引用 13

一句话总结

本研究利用星震学数据测试了演化恒星中泰勒不稳定性角动量（AM）输运的修正处方。尽管该模型预测的核心旋转速率低于纯流体动力学模型，并且与亚巨星观测结果的吻合度有所提高，但在所有测试的校准参数下，仍无法同时准确再现亚巨星和红巨星的核心旋转，表明当前的公式不足以解释星震学约束的全部范围。

ABSTRACT

Context: Asteroseismic measurements reveal that an unknown efficient angular momentum (AM) transport mechanism is needed for subgiant and red giant stars. A revised prescription for AM transport by the magnetic Tayler instability has been recently proposed as a possible candidate for such a missing mechanism. Results: The revised prescription for the transport by the Tayler instability leads to low core rotation rates after the main sequence that are in better global agreement with asteroseismic measurements than those predicted by models with purely hydrodynamic processes or with the original Tayler-Spruit dynamo. A detailed comparison with asteroseismic data shows that the rotational properties of at most two of the six subgiants can be correctly reproduced by models accounting for this revised magnetic transport process. This result is obtained independently of the value adopted for the calibration parameter in this prescription. We also find that this transport by the Tayler instability faces difficulties in simultaneously reproducing asteroseismic measurements available for subgiant and red giant stars. The low values of the calibration parameter needed to correctly reproduce the rotational properties of two of the six subgiants lead to core rotation rates during the red giant phase that are too high. Inversely, the higher values of this parameter needed to reproduce the core rotation rates of red giants lead to a very low degree of radial differential rotation before the red giant phase, which is in contradiction with the internal rotation of subgiant stars. Conclusions: In its present form, the revised prescription for the transport by the Tayler instability does not provide a complete solution to the missing AM transport revealed by asteroseismology of evolved stars.

研究动机与目标

• 测试修正的泰勒不稳定性角动量输运处方是否能够解释演化恒星内部旋转的星震学约束。
• 评估该模型再现亚巨星和红巨星核心与表面旋转速率的能力。
• 确定修正的磁性输运机制是否能够解释亚巨星阶段角动量输运效率降低以及红巨星分支阶段随后增大的现象。
• 评估不同恒星演化代码（日内瓦与MESA）下结果的稳健性。
• 识别当前泰勒不稳定性机制公式在解释观测到的旋转剖面时的物理局限性。

提出的方法

• 使用日内瓦恒星演化代码计算了采用修正泰勒不稳定性输运处方的恒星演化模型。
• 应用了依赖于无量纲校准参数α的修正输运处方，且该处方需要最小径向速度梯度才能激活。
• 将模型预测的混合模式频 splitting 与六颗亚巨星和红巨星的星震学测量结果进行比较。
• 通过使用MESA代码重新计算模型，对数值方法和输入物理参数的差异进行了结果一致性检验。
• 在广泛范围内改变校准参数α，以测试结果对该自由参数的敏感性。
• 分析了核心与表面旋转速率的演化，重点关注从亚巨星到红巨星阶段的过渡。

实验结果

研究问题

• RQ1 修正的泰勒不稳定性处方能否再现星震学测量中亚巨星观测到的低核心旋转速率？
• RQ2 修正的磁性输运机制是否能同时再现亚巨星和红巨星观测到的核心旋转速率？
• RQ3 校准参数α如何影响模型在不同演化阶段匹配星震学约束的能力？
• RQ4 模型预测的亚巨星阶段角动量输运效率下降是否与观测一致？
• RQ5 模型在红巨星阶段前预测的强径向速度梯度是否与观测到的亚巨星旋转剖面矛盾？

主要发现

• 修正的泰勒不稳定性处方预测的亚巨星核心旋转速率显著低于纯流体动力学模型，且与星震学测量结果的整体一致性更好。
• 无论校准参数α取值如何，最多仅有六颗亚巨星中的两颗能被采用修正泰勒不稳定性模型正确再现其旋转特性。
• 为匹配亚巨星数据而需采用的低α值，会导致红巨星阶段的核心旋转速率过高，与观测结果矛盾。
• 为匹配红巨星核心旋转速率而需采用的高α值，会导致红巨星阶段前径向速度梯度不足，与亚巨星旋转剖面的星震学约束矛盾。
• 这种矛盾的根源在于，修正机制预测在红巨星阶段角动量输运效率显著降低，因此必须在演化早期通过过度高效的输运来补偿。
• 结果在不同恒星演化代码下具有稳健性，因为使用MESA计算的模型也表现出相同的不一致，证实问题源于物理处方本身。