Shear mixing in stellar radiative zones I. Effect of thermal diffusion and chemical stratification
Shear mixing in stellar radiative zones I. Effect of thermal diffusion and chemical stratification
复制标题
恒星辐射区的剪切混合 I. 热扩散和化学分层的影响
DOI：
10.1051/0004-6361/201423655
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发表时间：
2014
影响因子：
6.5
通讯作者：
F. Lignieres
中科院分区：
文献类型：
--
作者：
V. Prat;F. Lignieres
上下文。由于唯象上的扩散系数，目前的恒星演化代码考虑了恒星辐射区中化学元素的湍流传输。最近的局部数值模拟(Prat&Lignieres 2013，A&A，551，L3)表明，径向旋转引起的径向湍流扩散系数满足dT‘0：058=Ri，与Zahn(1992，A&A，265,115)的模型定性一致。然而，该模型不适用于(I)相对于稳定热层结的差动旋转较强的情况，或(Ii)当化学层结具有显著的动力效应时，即在核燃烧对流核的外边界遇到的情况。目标。我们将我们的数值研究扩展到考虑化学分层和强剪切的影响，并将结果与恒星演化代码中使用的公式进行了比较。方法：研究方法。我们在Boussinesq近似下对稳定分层、均匀、剪切湍流进行了局部、直接的数值模拟。由于所谓的小Peclet数近似，即Boussinesq方程在这一区域的渐近发展，达到了恒星辐射区的典型的高热扩散区域。在此近似下，探讨了扩散系数对化学分层的依赖关系。结果。Maeder对Zahn模型在强剪切区域的扩展(Maeder 1995，A&A，299，84)不被我们的结果所支持，而我们的结果更好地描述了地球物理文献中的一个模型。对于化学分层效应，我们对扩散系数作为平均分子量梯度的函数的定量估计得到了公式DT‘0：45(0：12Ri)=Ri，它在弱切变区与Maeder&Meynet(1996，A&A，313,140)的模型相容，但与Maeder(1997，A&A，321,134)的模型不相容。
Context. Turbulent transport of chemical elements in radiative zones of stars is considered in current stellar evolution codes thanks to phenomenologically derived di usion coe cients. Recent local numerical simulations (Prat & Lignieres 2013, A&A, 551, L3) suggest that the coe cient for radial turbulent di usion due to radial di erential rotation satisfies Dt ’ 0:058= Ri, in qualitative agreement with the model of Zahn (1992, A&A, 265, 115). However, this model does not apply (i) when di erential rotation is strong with respect to stable thermal stratification or (ii) when chemical stratification has a significant dynamical e ect, a situation encountered at the outer boundary of nuclear-burning convective cores. Aims. We extend our numerical study to consider the e ects of chemical stratification and of strong shear, and compare the results with prescriptions used in stellar evolution codes. Methods. We performed local, direct numerical simulations of stably stratified, homogeneous, sheared turbulence in the Boussinesq approximation. The regime of high thermal di usivities, typical of stellar radiative zones, is reached thanks to the so-called smallPeclet-number approximation, which is an asymptotic development of the Boussinesq equations in this regime. The dependence of the di usion coe cient on chemical stratification was explored in this approximation. Results. Maeder’s extension of Zahn’s model in the strong-shear regime (Maeder 1995, A&A, 299, 84) is not supported by our results, which are better described by a model found in the geophysical literature. As regards the e ect of chemical stratification, our quantitative estimate of the di usion coe cient as a function of the mean gradient of mean molecular weight leads to the formula Dt ’ 0:45 (0:12 Ri )=Ri, which is compatible in the weak-shear regime with the model of Maeder & Meynet (1996, A&A, 313, 140) but not with Maeder’s (1997, A&A, 321, 134).