Star-in-a-box simulations of fully convective stars
Star-in-a-box simulations of fully convective stars
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DOI：
10.1051/0004-6361/202040049
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发表时间：
2020-12
影响因子：
6.5
通讯作者：
P. Käpylä
中科院分区：
文献类型：
--
作者：
P. Käpylä
上下文。质量小于0.35 M⊙的主序晚型恒星是完全对流的。目标。目标是研究对流、微分旋转和发电机作为完全对流恒星旋转的函数。方法。采用星盒模型进行了三维流体力学和磁流体力学数值模拟，其中一颗球形恒星浸没在笛卡尔立方体内。该模型对应于一颗0.2 M⊙主序M5矮星。研究了4.3 - 430 d之间的旋转周期（Prot）范围。结果。Prot = 430天的慢旋转模型产生反太阳微分旋转，赤道慢，两极快，主要是轴对称的准稳定大尺度磁场。对于中间自转（Prot = 144和43天），差旋是类似太阳的（赤道快，两极慢），大尺度磁场大多是轴对称的，准平稳或循环。后者发生的参数范围与其他在球壳中的数值研究相似，周期与在旋转周期约为100天的完全对流恒星中观测到的周期相似。在快速旋转区，微分旋转较弱，大尺度磁场越来越非轴对称，以m = 1模式占主导地位。这种大尺度非轴对称场也表现出方位发电机波。结论。盒中恒星模型的结果与球壳中部分对流的晚型恒星的模拟结果一致，因为从科里奥利数（逆罗斯比）来看，微分旋转和发电机状态的转变发生在相似的旋转状态。部分对流和完全对流恒星之间的这种相似性表明，产生微分旋转和大尺度磁性的过程对恒星的几何形状不敏感。
Context. Main-sequence late-type stars with masses of less than 0.35 M⊙ are fully convective. Aims. The goal is to study convection, differential rotation, and dynamos as functions of rotation in fully convective stars. Methods. Three-dimensional hydrodynamic and magnetohydrodynamic numerical simulations with a star-in-a-box model, in which a spherical star is immersed inside of a Cartesian cube, are used. The model corresponds to a 0.2 M⊙ main-sequence M5 dwarf. A range of rotation periods (Prot) between 4.3 and 430 d is explored. Results. The slowly rotating model with Prot = 430 days produces anti-solar differential rotation with a slow equator and fast poles, along with predominantly axisymmetric quasi-steady large-scale magnetic fields. For intermediate rotation (Prot = 144 and 43 days) the differential rotation is solar-like (fast equator, slow poles), and the large-scale magnetic fields are mostly axisymmetric and either quasi-stationary or cyclic. The latter occurs in a similar parameter regime as in other numerical studies in spherical shells, and the cycle period is similar to observed cycles in fully convective stars with rotation periods of roughly 100 days. In the rapid rotation regime the differential rotation is weak and the large-scale magnetic fields are increasingly non-axisymmetric with a dominating m = 1 mode. This large-scale non-axisymmetric field also exhibits azimuthal dynamo waves. Conclusions. The results of the star-in-a-box models agree with simulations of partially convective late-type stars in spherical shells in that the transitions in differential rotation and dynamo regimes occur at similar rotational regimes in terms of the Coriolis (inverse Rossby) number. This similarity between partially and fully convective stars suggests that the processes generating differential rotation and large-scale magnetism are insensitive to the geometry of the star.