流体动力学壳层对流模型中的Rayleigh-Taylor指形不稳定性混合
Rayleigh-Taylor finger instability mixing in hydrodynamic shell convection models
摘要：
由于瑞利-泰勒不稳定性的结果，由成分梯度驱动的恒星中的混合过程是不可预见的。它们只得到恒星对流的流体动力学研究的支持。我们发现，这种混合发生在我们的多维流体动力学壳对流模型的对流区的底部边缘以下。它在偏离中心的核燃烧产生的界面上工作，在那里，具有较高平均分子量的密度较小的气体位于具有较低平均分子量的密度较大的气体之上。我们讨论了在各种条件下的混合流体动力学对流模型的基础上恒星的演化计算的核心氦闪光在1.25 Msun的恒星，核心碳闪光在9.3 Msun的恒星，和氧燃烧壳的恒星质量为23 Msun。我们用欧拉流体动力学程序HERAKLES在二维和三维空间模拟了恒星演化不同阶段壳层对流的流体动力学行为。为此目的的初始模型是由国家的最先进的恒星演化代码，即GARSTEC，STAREVOL，和TYCHO的核心氦闪光，核心碳闪光，和氧壳燃烧，分别获得。我们的大部分分析是在核心氦闪光的峰值覆盖约250对流周转时间尺度的二维流体动力学模型壳对流。这种混合表现为高密度和冷指的形式，富含较高平均分子量的物质，起源于对流壳层下边界的密度波动，并向下“射”入核心。它们是由于平均分子量梯度为负而在对流区下边界处产生的瑞利-泰勒不稳定性。当平均分子量梯度为正时，它们不出现。
英文摘要：
Mixing processes in stars driven by composition gradients as a result of the Rayleigh-Taylor instability are not anticipated. They are supported only by hydrodynamic studies of stellar convection. We find that such mixing occurs below the bottom edge of convection zones in our multidimensional hydrodynamic shell convection models. It operates at interfaces created by off-center nuclear burning, where less dense gas with higher mean molecular weight is located above denser gas with a lower mean molecular weight. We discuss the mixing under various conditions with hydrodynamic convection models based on stellar evolutionary calculations of the core helium flash in a 1.25 Msun star, the core carbon flash in a 9.3 Msun star, and of oxygen burning shell in a star with a mass of 23 Msun. We simulate the hydrodynamic behavior of shell convection during various phases of stellar evolution with the Eulerian hydrodynamics code HERAKLES in two and three spatial dimensions. Initial models for this purpose are obtained by state-of-the-art stellar evolutionary codes, namely GARSTEC, STAREVOL, and TYCHO for the core helium flash, core carbon flash, and oxygen shell burning, respectively. Most of our analysis is performed for two-dimensional hydrodynamic models of shell convection during the core helium flash at its peak covering approximately 250 convective turnover timescales. The mixing manifests itself in the form of overdense and cold fingers enriched with matter of higher mean molecular weight, originating from density fluctuations at the lower boundary of the convective shell, and "shooting" down into the core. They result from the Rayleigh-Taylor instability at the lower convection zone boundary due to a negative mean molecular weight gradient. They do not appear when the mean molecular weight gradient is positive.
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