TURBULENT CELLS IN STARS: FLUCTUATIONS IN KINETIC ENERGY AND LUMINOSITY
TURBULENT CELLS IN STARS: FLUCTUATIONS IN KINETIC ENERGY AND LUMINOSITY
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恒星中的湍流细胞：动能和光度的波动
DOI：
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发表时间：
2010
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通讯作者：
C. Meakin
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作者：
W. Arnett;C. Meakin;C. Meakin
壳层氧气燃烧的三维 (3D) 流体动力学模拟表现出湍流动能的突发性、周期性波动。这些被证明是由于对流单元的普遍不稳定性，仅需要局部加热或冷却源。在使用混合长度理论的恒星演化模拟中，这种波动被证明受到抑制。定量上类似的行为也发生在洛伦兹的对流卷（细胞）模型中，众所周知，洛伦兹的对流卷（细胞）模型具有一个奇怪的吸引子，它会引起速度时间的混沌波动，正如我们所展示的，光度。模拟研究表明，洛伦兹对流卷的行为可能类似于对流中细胞的行为。我们研究了这种最简单近似的一些含义，并提出了改进路径。使用洛伦兹模型作为对流细胞的代表，对流层的多细胞模型给出了总光度波动，这暗示了不规则变量（红巨星和超巨星），以及半规则渐近巨星分支变量中的长次级周期特征。这种“τ机制”是恒星变异性的一个新来源，它本质上是非线性的（在线性稳定性分析中看不到），并且与湍流的间歇性密切相关。它已经隐含在 Woodward 等人的 3D 全局模拟中。这种波动行为可以在 CNeOSi 燃烧壳的扩展二维模拟中看到，并且可能会导致不稳定，从而导致核心塌陷超新星在塌缩之前爆发。
Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning exhibit bursty, recurrent fluctuations in turbulent kinetic energy. These are shown to be due to a general instability of the convective cell, requiring only a localized source of heating or cooling. Such fluctuations are shown to be suppressed in simulations of stellar evolution which use the mixing-length theory. Quantitatively similar behavior occurs in the model of a convective roll (cell) of Lorenz, which is known to have a strange attractor that gives rise to chaotic fluctuations in time of velocity and, as we show, luminosity. Study of simulations suggests that the behavior of a Lorenz convective roll may resemble that of a cell in convective flow. We examine some implications of this simplest approximation and suggest paths for improvement. Using the Lorenz model as representative of a convective cell, a multiple-cell model of a convective layer gives total luminosity fluctuations which are suggestive of irregular variables (red giants and supergiants), and of the long secondary period feature in semiregular asymptotic giant branch variables. This “τ-mechanism” is a new source for stellar variability, which is inherently nonlinear (unseen in linear stability analysis), and one closely related to intermittency in turbulence. It was already implicit in the 3D global simulations of Woodward et al. This fluctuating behavior is seen in extended two-dimensional simulations of CNeOSi burning shells, and may cause instability which leads to eruptions in progenitors of core-collapse supernovae prior to collapse.