红巨星盒中恒星模拟的平均场解释

中文摘要 AI 辅助

背景：红巨星磁场的起源和发电机机制仍未完全理解。目的：我们旨在用平均场发电机模型解释红巨星全局三维模拟的发电机行为。方法：我们使用由三维模拟提取的差动旋转剖面约束的平均场模型。我们进行了$\alpha^2$、$\alpha\Omega$和$\alpha^2\Omega$平均场发电机模拟，改变$\alpha$效应的强度和差动旋转。结果：平均场模型可以再现几个三维运行的增长率。对于缓慢旋转的三维情况，大尺度磁场的形态比快速旋转的情况再现得更好。更快的旋转增强了发电机作用，也改变了发电机的主导模式。快速旋转的三维运行主要产生非轴对称赤道偶极子，而不是较慢旋转时的轴对称场。即使在没有差动旋转的情况下，平均场模型也是超临界的，表明存在$\alpha^2$发电机作用。相比之下，使用$\alpha\Omega$近似的模型需要足够强的差动旋转才能变得超临界。结论：我们的结果表明红巨星中的磁场需要发电机作用。在我们的平均场运行中，差动旋转加速了磁衰减，不支持红巨星中存在持久化石磁场的观点。

英文摘要

Context: The origin of magnetic fields and the dynamo mechanism in red giants are still not fully understood. Aims: We aim to interpret the dynamo behaviour of global 3D simulations of red giants using mean-field dynamo models. Methods: We use mean-field models constrained by the differential rotation profile extracted from 3D simulations. We perform $α^2$, $αΩ$, and $α^2Ω$ mean-field dynamo simulations, varying the strength of the $α$ effect and the differential rotation. Results: The mean-field models can reproduce the growth rate of several 3D runs. The morphology of the large-scale magnetic field is better reproduced for the slowly rotating 3D cases than for the rapidly rotating ones. Faster rotation enhances dynamo action and it also modifies the dominant mode of the dynamo. Rapidly rotating 3D runs produce predominantly non-axisymmetric equatorial dipoles instead of axisymmetric fields at slower rotation. Mean-field models are supercritical even in the absence of differential rotation, indicating $α^2$ dynamo action. By contrast, models using the $αΩ$ approximation require sufficiently strong differential rotation to become supercritical. Conclusions: Our results suggest that the magnetic field in red giants requires dynamo action. In our mean-field runs, differential rotation speeds up the magnetic decay, disfavouring the idea of a persistent fossil magnetic field in red giants.

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13 pages, 15 figures. Submitted to Astronomy & Astrophysics