Enhanced Mixing in Magnetized Fingering Convection, and Implications for Red Giant Branch Stars
Enhanced Mixing in Magnetized Fingering Convection, and Implications for Red Giant Branch Stars
复制标题
磁化指状对流中的增强混合及其对红巨星分支星的影响
作者：
P. Harrington;P. Garaud
双扩散对流在地球物理背景下已经得到了很好的研究，但是对恒星或行星内部特征的详细研究直到最近才成为可能。由于大多数天体物理流体都是导电的，磁场有可能在增强或抑制双扩散对流方面发挥作用，但迄今为止还没有对这种可能性进行数值研究。本文通过理论工作和直接数值模拟相结合的方法，研究了垂直背景磁场（与重力轴对齐）对指指（热盐）对流线性稳定性和非线性饱和的影响。我们发现垂直磁场使手指沿垂直方向变硬，这对增强垂直混合有显著的效果。我们提出了一个简单的磁化指指对流混合的分析模型，并认为磁效应可能有助于解释理论和观测到的低质量红巨星分支恒星混合率之间的差异。我们的研究结果的其他含义也进行了讨论。
Double-diffusive convection has been well studied in geophysical contexts, but detailed investigations of the regimes characteristic of stellar or planetary interiors have only recently become feasible. As most astrophysical fluids are electrically conducting, it is possible that magnetic fields play a role in either enhancing or suppressing double-diffusive convection, but to date there have been no numerical investigations of such possibilities. Here we study the effects of a vertical background magnetic field (aligned with the gravitational axis) on the linear stability and nonlinear saturation of fingering (thermohaline) convection, through a combination of theoretical work and direct numerical simulations. We find that a vertical magnetic field rigidifies the fingers along the vertical direction, which has the remarkable effect of enhancing vertical mixing. We propose a simple analytical model for mixing by magnetized fingering convection, and argue that magnetic effects may help explain discrepancies between theoretical and observed mixing rates in low-mass red giant branch stars. Other implications of our findings are also discussed.