转动恒星结构与演化研究

Structure and Evolution of Rotating Stars

恒星的自转 ,是恒星结构和演化理论的难点。近年来有许多观测事实 ,特别是早型大质量星的观测事实 ,预示恒星的自转效应可能引起恒星内部的物质向外转移 ,造成恒星表面一些元素丰度超丰 ,并且对恒星结构和演化产生重要影响 ,因此 ,恒星的自转问题受到了越来越多的关注。考虑自转效应后 ,恒星结构和演化模型将是二维模型 ,本文综述了诸多作者如何将二维的恒星结构和演化模型简化为一维模型。作者在研究了以上作者的简化方法后 ,提出了一种比较简单的新方法。这种方法基于如下假设 :假设在等势面上的温度 ,密度 ,压强 ,光度 ,化学组成和角速度等物理和化学量近似于均匀分布 ,并且这些量与等价球面上的量相同。 (等价球面是假想的球面 ,它包围的体积与等势面包围的体积相等。)我们在等价球面上推出新的转动恒星结构和演化方程 ,构造出新的演化模型。这个模型与不考虑转动效应的演化模型相比 ,有以下变化 :流体静力学平衡方程变化 ;辐射温度梯度变化 ,并引起对流判据变化 ;星风物质损失和角动量损失增大。作为转动恒星结构和演化模型的应用 ,我们研究了中 ,小质量星中心氦燃烧阶段在赫罗图中的演化轨迹发生来回摆动 (又称为蓝回绕 )的物理机制问题。有诸多作者曾经研究了可以影响蓝回绕的各种因素。

The star rotation is a difficult problem in the theory of the stellar structure and evolution. Recent observational data, especially observational data for the early type stars have indicated that some efficient mixing processes might be induced by the rotation. Thus, the study of the evolution of rotating stars has attracted more and more attention in the last decades. The stellar structure and evolution model will be two dimensional due to the effects of the rotation. Many scientists have tried to simplify the two dimensional stellar structure and evolution model to one dimensional. Their ways of dealing this problem are reviewed, and a relative simple way is proposed based on the following assumption: physical quantities such as the temperature, density, pressure, luminosities, chemical composition and angular velocities are nearly the same on the equipotential surfaces, and they are equal to their counterparts on the equivalent sphere. We have derived the new structure and evolution equations for rotating stars by using the quantities of an point on the equivalent sphere, and construced a new model. Comparing with the non-rotating model, it has the following changes: the change in the hydrostatic equation; the change in the radiation temperature gradient and the criterion of the convection, and the enhancement of the stellar wind and angular momentum loss. As the application of our new rotating model,we have studied the main factors which affect the formation and extension of the blue loops for the middle and low mass stars. It is interesting to notice that although the rotation and overshooting can both affect the length of the blue loops, their effects are appreciably different. Hence, a comparison of the differences in the inner structure of a star affected by the rotation and overshooting can isolate those factors, which play a primary role in the formation and extension of the blue loops. We have found that the feature of the blue loops is correlated to the variation of the total energy production rate of the star during the core He-burning. When the variation of the total energy production rate is contributed mainly by the H-burning shell, the formation and extension of the blue loops is dependent on the temperature and the hydrogen profile in the μ-gradient region and the inward penetration of the outer convective zone. When the variation of the total energy production rate is contributed by both of the H-burning shell and the core He-burning, the formation and extension of the blue loops is dependent not only on the properties of the μ-gradient region, but also on both of the size of the He core and the temperature in the region of the He core. In the present paper, some physical effects of the star rotation are discussed.