arXiv 1202.43492012-02-20astro-ph.SR

RGB星对流包层下过冲区湍流对流的化学混合

Chemical mixing by turbulent convection in the overshooting region below the convective envelope of RGB stars

X. -J. Lai, Y. Li

备注：: 20 pages, 15 figures

摘要：

基于湍流对流模型（TCM），研究了中等质量恒星对流包层底部过冲区的化学混合，重点讨论了其对Hertzsprung-Russell（HR）图中蓝环形成和扩展的影响.在红巨星分支（RGB）阶段采用扩散混合模型。蓝环的性质是通过改变H燃烧壳层上方的元素分布而改变的，这是由于恒星模型沿RGB向上演化时底部过冲区的不完全混合造成的。元素轮廓的这种改变将导致H燃烧壳层正上方区域的不透明度增加和外部均匀对流区的不透明度降低，这将导致H壳层核光度$L_{H}$在恒星模型从RGB尖端向底部演化时迅速降低，在恒星包层中将获得更弱和更小的对流区。这有助于形成更长的蓝色循环。蓝环的延伸对扩散混合模型和TCM的参数$(C_{X}$和$α_{TCM})$非常敏感。结果主要表明：1）与经典混合长度理论模型的结果相比，不同$C_{X}$和$α_{TCM}$值组合下得到的蓝环长度均有所增加，且蓝环长度随参数$C_{X}$和$α_{TCM}$值的增加而增加; 2）扩散混合模型可以显著延长恒星模型在HR图蓝边的停留时间，尽管7 $M_{\odot}$星的蓝环长度在经典混合模型和扩散混合模型之间的差异不太显著;

英文摘要：

Based on the turbulent convection model (TCM), we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediate-mass stars, focusing on its influence on the formation and extension of blue loops in the Hertzsprung-Russell (HR) diagram. A diffusive mixing model is adopted during the Red Giant Branch (RGB) phase. The properties of the blue loop are changed by modification of the element profiles above the H-burning shell, which results from the incomplete mixing in the bottom overshooting region when stellar model evolves up along the RGB. Such modification of the element profiles will lead to an increase of the opacity in the region just above the H-burning shell and a decrease of the opacity in the outer homogeneous convection zone, which will result in a quick decrease of the H-shell nuclear luminosity $L_{H}$ when the stellar model evolves from the RGB tip to its bottom and, finally, a much weaker and smaller convection zone will be obtained in the stellar envelope. This helps to form a longer blue loop. The extension of the blue loop is very sensitive to the parameters $(C_{X}$ and $α_{TCM})$ of the diffusive mixing model and of the TCM. The results mainly show that: 1) comparing to the results of the classical model with the mixing-length theory, the lengths of the obtained blue loops with different combinations of the values of $C_{X}$ and $α_{TCM}$ are all increased and the length of the blue loop increases with the values of parameters $C_{X}$ and $α_{TCM}$; 2) the diffusive mixing model can significantly extend the time of stellar models lingering on the blue side of the HR diagram, even though the length of the blue loop for the 7$M_{\odot}$ star has a less prominent difference between the classical and diffusive mixing model;

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