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Angular momentum and chemical transport by azimuthal magnetorotational instability in radiative stellar interiors
Astronomy & Astrophysics ( IF 6.5 ) Pub Date : 2024-02-29 , DOI: 10.1051/0004-6361/202347672 Domenico G. Meduri , Laurène Jouve , François Lignières
Astronomy & Astrophysics ( IF 6.5 ) Pub Date : 2024-02-29 , DOI: 10.1051/0004-6361/202347672 Domenico G. Meduri , Laurène Jouve , François Lignières
Context. The transport of angular momentum and chemical elements within evolving stars remains poorly understood. Asteroseismic and spectroscopic observations of low-mass main sequence stars and red giants reveal that their radiative cores rotate orders of magnitude slower than classical predictions from stellar evolution models and that the abundances of their surface light elements are too small. Magnetohydrodynamic (MHD) turbulence is considered a primary mechanism to enhance the transport in radiative stellar interiors but its efficiency is still largely uncertain.Aims. We explore the transport of angular momentum and chemical elements due to azimuthal magnetorotational instability, one of the dominant instabilities expected in differentially rotating radiative stellar interiors.Methods. We employed 3D MHD direct numerical simulations in a spherical shell of unstratified and stably stratified flows under the Boussinesq approximation. The background differential rotation was maintained by a volumetric body force. We examined the transport of chemical elements using a passive scalar.Results. We provide evidence of magnetorotational instability for purely azimuthal magnetic fields in the parameter regime expected from local and global linear stability analyses. Without stratification and when the Reynolds number Re and the background azimuthal field strength are large enough, we observed dynamo action driven by the instability at values of the magnetic Prandtl number Pm in the range 0.6 − 1, which is the smallest ever reported in a global setup. When considering stable stratification at Pm = 1, the turbulence is transitional and becomes less homogeneous and isotropic upon increasing buoyancy effects. The transport of angular momentum occurs radially outward and is dominated by the Maxwell stresses when stratification is large enough. We find that the turbulent viscosity decreases when buoyancy effects strengthen and scales with the square root of the ratio of the reference rotation rate Ωa to the Brunt–Väisälä frequency N. The chemical turbulent diffusion coefficient scales with stratification similarly to the turbulent viscosity, but is lower in amplitude so that the transport of chemicals is slower than the one of angular momentum, in agreement with recent stellar evolution models of low-mass stars.Conclusions. We show that the transport induced by azimuthal magnetorotational instability scales somewhat slowly with stratification and may enforce rigid rotations of red giant cores on a timescale of a few thousand years. In agreement with recent stellar evolution models of low-mass stars, the instability transports chemical elements less efficiently than angular momentum.
中文翻译：
辐射恒星内部方位角磁旋转不稳定性的角动量和化学输运
语境。演化中的恒星内部角动量和化学元素的传输仍然知之甚少。对低质量主序星和红巨星的星震和光谱观测表明，它们的辐射核心旋转速度比恒星演化模型的经典预测慢几个数量级，而且它们表面轻元素的丰度太小。磁流体动力（MHD）湍流被认为是增强辐射恒星内部传输的主要机制，但其效率仍然很大程度上不确定。目标。我们探索了由于方位角磁旋转不稳定性引起的角动量和化学元素的传输，这是差异旋转辐射恒星内部预期的主要不稳定性之一。方法。我们在 Boussinesq 近似下对非分层和稳定分层流的球壳进行了 3D MHD 直接数值模拟。背景差动旋转由体积力维持。我们使用被动标量检查了化学元素的传输。结果。我们提供了局部和全局线性稳定性分析预期的参数范围内纯方位磁场的磁旋转不稳定性的证据。在没有分层的情况下，当雷诺数Re和背景方位角场强度足够大时，我们观察到由磁普朗特数Pm值在0.6-1范围内的不稳定性驱动的发电机作用，这是全球有史以来报道的最小的。设置。当考虑 Pm = 1 时的稳定分层时，湍流是过渡性的，并且随着浮力效应的增加而变得不那么均匀和各向同性。当分层足够大时，角动量的传输发生在径向向外，并且由麦克斯韦应力主导。我们发现，当浮力效应增强时，湍流粘度会降低，并且与参考旋转速率 Ω a与 Brunt-Väisälä 频率N之比的平方根成比例。化学湍流扩散系数与层化成比例，类似于湍流粘度，但其振幅较低，因此化学物质的传输比角动量的传输慢，这与最近的低质量恒星的恒星演化模型一致。结论。我们表明，方位角磁旋转不稳定性引起的输运随着分层的变化而变化，并且可能会在几千年的时间尺度上强制红巨星核心的刚性旋转。与最近的低质量恒星的恒星演化模型一致，不稳定性传输化学元素的效率低于角动量。
更新日期：2024-02-29
中文翻译：
辐射恒星内部方位角磁旋转不稳定性的角动量和化学输运
语境。演化中的恒星内部角动量和化学元素的传输仍然知之甚少。对低质量主序星和红巨星的星震和光谱观测表明，它们的辐射核心旋转速度比恒星演化模型的经典预测慢几个数量级，而且它们表面轻元素的丰度太小。磁流体动力（MHD）湍流被认为是增强辐射恒星内部传输的主要机制，但其效率仍然很大程度上不确定。目标。我们探索了由于方位角磁旋转不稳定性引起的角动量和化学元素的传输，这是差异旋转辐射恒星内部预期的主要不稳定性之一。方法。我们在 Boussinesq 近似下对非分层和稳定分层流的球壳进行了 3D MHD 直接数值模拟。背景差动旋转由体积力维持。我们使用被动标量检查了化学元素的传输。结果。我们提供了局部和全局线性稳定性分析预期的参数范围内纯方位磁场的磁旋转不稳定性的证据。在没有分层的情况下，当雷诺数Re和背景方位角场强度足够大时，我们观察到由磁普朗特数Pm值在0.6-1范围内的不稳定性驱动的发电机作用，这是全球有史以来报道的最小的。设置。当考虑 Pm = 1 时的稳定分层时，湍流是过渡性的，并且随着浮力效应的增加而变得不那么均匀和各向同性。当分层足够大时，角动量的传输发生在径向向外，并且由麦克斯韦应力主导。我们发现，当浮力效应增强时，湍流粘度会降低，并且与参考旋转速率 Ω a与 Brunt-Väisälä 频率N之比的平方根成比例。化学湍流扩散系数与层化成比例，类似于湍流粘度，但其振幅较低，因此化学物质的传输比角动量的传输慢，这与最近的低质量恒星的恒星演化模型一致。结论。我们表明，方位角磁旋转不稳定性引起的输运随着分层的变化而变化，并且可能会在几千年的时间尺度上强制红巨星核心的刚性旋转。与最近的低质量恒星的恒星演化模型一致，不稳定性传输化学元素的效率低于角动量。
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