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题名	RGB和AGB星的外壳湍流运动及引起的元素混合效应
作者	赖祥军
学位类别	博士
答辩日期	2011-05-26
授予单位	中国科学院研究生院（云南天文台）
授予地点	北京
导师	李焱
关键词	湍流模型 RGB和AGB星 湍流运动 元素混合 蓝回绕
其他题名	The turbulence in outer envelope of RGB and AGB stars and the effect of chemical mixing caused by it
学位专业	天体物理
中文摘要	红巨星分支(RGB)星和渐进巨星分支(AGB)星是中小质量恒星从零龄主序演化到后期时出现的亮而红的巨星。它们的有效温度比较低，而光度比较大，这使得它们的外部壳层处于对流状态，而且非常厚。本论文中，我们根据新的二阶湍流模型研究了RGB和AGB星中的湍流特征，并和经典的混合长模型的结果做了比较。另外，还用该模型研究了湍流运动引起的元素混合对恒星结构和演化的影响，同时，利用造父变星脉动不稳定带和红蓝星数比的观测结果对湍流模型以及模型参数进行了检验和限制。 研究发现，沿着Hayashi线向上分布的RGB和AGB星中的湍流运动是越来越激烈的。湍流模型得到的湍流热通量及湍流速度通常比混合长模型给出的结果要大，当减小湍流模型中混合长参数的值使得湍流模型和混合长模型给出几乎相同的湍流速度轮廓时，湍流模型得到的超绝热温度梯度的区域向内延展，恒星模型的有效温度将会降低。另外，沿着Hayashi线向上RGB和AGB星中的顶部和底部超射区中湍流速度衰减的e-folding距离都是减小的，但变化范围不大。还有，在湍流运动逐渐衰弱的超射区中湍动能和压强之间几乎存在线性关系，该关系的斜率主要由湍流模型决定，而其常数项主要由恒星模型决定。 考虑RGB星中外部对流壳层底部超射区中的元素混合(按扩散方式处理)效应后，结果表明，在RGB演化阶段中恒星模型的外部对流壳层将会向内增大，而且恒星模型在RGB顶部的有效温度有轻微的增加而光度有轻微的减小。当恒星模型从RGB顶部再次演化到底部时，外部对流壳层将变得非常薄从而使得恒星模型向蓝端方向增加蓝回绕幅度的趋势演化，而且蓝回绕的幅度都随扩散模型中的扩散参数CX和湍流模型中的混合长参数aTCM的增加而增加，同时恒星模型在中心氦燃烧时处在蓝端的时间也随它们的增加而增加。所得结果与造父脉动不稳定带和蓝红星数比的观测结果相符合，而且这两种观测数据对5M☉恒星模型中的CX和aTCM的取值都分别限定在同一范围中：0.5
英文摘要	Red giant branch(RGB) and asymptotic giant branch(AGB) stars are the luminous and red giants in late periods of stellar evolution from the main sequence stage of low to intermediate-mass stars. Their effective temperatures are rather low, however, their luminosities are rather high, which leads to the fact that their outer envelope is convective and the convective envelopes are enormous in mass or radius. Based on a new second-order turbulent convection model(TCM), we have studied the characteristics of turbulent convection in RGB and AGB stars, and made comparison with the results of the classical mixing length theory(MLT). Furthermore, we have utilized the TCM to research the effect of chemical mixing caused by the turbulent convective motion on stellar structure and evolution. At the same time, the TCM and some free parameters have been tested and confined according to the observational data on the Cepheid instability strip and the number ratio of blue to red evolved stars. It is found that, the convective motions become stronger and stronger when RGB and AGB stellar models are located up along the Hayashi line. The TCM usually gives larger convective heat fluxes and larger turbulent velocity than the MLT does, however, when the turbulent velocity is adjusted to close to that of the MLT by decreasing the value of the mixing length parameter in the TCM, the superadiabatic convection zone will be much more extended inward and a lower effective temperature of the stellar model will be obtained. Besides, the e-folding lengths of the turbulent velocity in both the top and bottom overshooting regions decrease as the stellar model is located up along the Hayashi line, but both the extents of the decrease are not obvious. Moreover, in the overshooting regions there is a nearly linear relationship between the turbulent kinetic energy and the pressure. And the slope of the relationship is mainly determined by the TCM, however, the constant term in it is mainly determined by the stellar model. After taking the effect of the chemical mixing(disposed as a diffusive process) in the bottom overshooting region of the outer convective envelpe into account, it is found that in the stage of the RGB the convective envelope of the stellar model will be enlarged inward, and the effective temperature and the luminosity of the stellar model at the RGB tip will be increased and decreased a little, respectively. When the stellar model evolves from the RGB tip down to its bottom again, the convective envelope will be suppressed to a rather small extent, which leads to a trend that the stellar model will evolve blueward to increase the length of blue loop. And the blue loop will become longer and longer as both the parameter CX in the diffusive mixing model and aTCM in the TCM increase, furthermore, the time interval of the stellar model staying around the blue end of the blue loop will also be increased as both the CX and aTCM increase. The obtained results are in agreement with the observations of the Cepheid instability strip and the number ratio of blue to red evolved stars. And both the two observations respectively confine the CX and aTCM in a same range: 0.5
语种	中文
学科主题	天文学
公开日期	2016-05-03
页码	96
内容类型	学位论文
源URL	[http://ir.ynao.ac.cn/handle/114a53/5333]
专题	云南天文台_恒星物理研究组
推荐引用方式 GB/T 7714	赖祥军. RGB和AGB星的外壳湍流运动及引起的元素混合效应[D]. 北京. 中国科学院研究生院（云南天文台）. 2011.

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