太阳附近恒星Si元素丰度的非局部热动平衡分析

发布时间：2018-03-21 05:32

本文选题：银河系　切入点：恒星　出处：《河北师范大学》2012年硕士论文　论文类型：学位论文

【摘要】：银河系的形成和演化是非常重要的天体物理问题之一，不同年龄的恒星，其化学元素丰度能够提供银河系不同演化阶段的化学组成。恒星元素丰度是追踪银河系化学演化很好的探针，因此元素丰度的观测分析一直是天体物理研究的一个重要课题。本文主要研究太阳附近恒星中的Si元素丰度，选择Si元素进行研究的原因有两个：一是因为Si元素作为典型的元素一般认为产生于星系形成早期的SN II超新星爆发过程，因此可以作为示踪元素来研究星系早期的化学演化；二是由于Si元素属于恒星中丰度较高的元素，是晚型恒星中重要的电子来源。高分辨率、高信噪比的恒星光谱是在2001至2003年间的利用Calar Alto天文台的DSAZ2.2米望远镜及其附属的FOCES光纤折轴光谱仪观测得到的。目前关于Si元素的丰度分析一般都是在局部热动平衡（LTE）效应的假定下进行的，，但对于那些恒星大气明显偏离LTE状态的恒星来说，丰度分析需要考虑非局部热动平衡（NLTE）效应的影响。我们的工作主要是通过观测得到太阳附近的166颗矮星的光谱，建立恒星大气模型，采用光谱综合方法，分别在局部热动平衡及非局部热动平衡两个假定下分析得到样本恒星的Si元素丰度，目的是研究NLTE效应对Si元素丰度确定的影响。我们分别采用运动学和丰度判据（[Mg/Fe]）来划分样本星的星族成分，研究了薄盘、厚盘及晕星等不同星族成分中[Si/Fe]的演化趋势。论文的第一部分是引言，主要介绍了恒星中Si元素丰度的观测研究现状；第二部分给出了观测样本和数据处理过程；第三部分是大气模型和恒星参数的确定；第四部分是Si的原子模型和非局部热动平衡的影响；第五部分给出了Si丰度的结果及对结果的分析；第六部分是结果讨论；最后第七部分是结论。通过分析，我们得到了以下几个重要结论：（1）NLTE对Si丰度的影响是与恒星有效温度相关的，热星的NLTE的修正较大。（2）NLTE效应对不同电离态的Si元素的丰度影响是不同的，其对中性原子Si I谱线的修正很小几乎可以忽略，但对一次电离的Si II谱线的丰度修正平均达到-0.06dex。这使得考虑NLTE效应后由Si I线和Si II线得到的丰度差由原来LTE下的0.07dex减小为0.02dex，表明考虑NLTE修正是十分必要的。（3）贫金属星中Si元素丰度随着[Fe/H]的增加呈减小趋势，但富金属星的[Si/Fe]随[Fe/H]的增加反而有轻微的上升趋势。（4）厚盘星的[Si/Fe]明显高于薄盘星，但并没有严格的区分薄盘和厚盘的明确界限，既存在一些运动学性质属于薄盘而元素丰度属于厚盘的恒星，也存在一些运动学性质属于厚盘而元素丰度属于薄盘的恒星。（5）尽管Si和Mg都是典型的元素，在星系演化早期都是大质量SNe II超新星爆发的产物，但在星系演化晚期，Si和Mg可能有不同的核合成机制，Si丰度可能部分来自于SNe Ia和AGB星的贡献。
[Abstract]:The formation and evolution of the Milky way Galaxy is one of the most important astrophysical problems, stars of different ages, Its chemical abundance can provide the chemical composition of the Milky way at different stages of evolution. Stellar abundance is a good probe for tracking the chemical evolution of the Milky way. Therefore, the observational analysis of element abundance has been an important subject in astrophysics. In this paper, we study the abundance of Si in stars near the sun. There are two reasons for choosing Si to study. One is that Si, as a typical element, is generally thought to have originated from the SN II supernova burst process in the early stage of galaxy formation, so it can be used as a tracer element to study the chemical evolution of the galaxy in the early stage. The second reason is that Si is an important electron source in late type stars because it is a high abundance element in stars. The high signal-to-noise ratio (SNR) spectra of stars were observed from 2001 to 2003 using the DSAZ2.2 meter telescope of the Calar Alto Observatory and its affiliated FOCES fiber-optic fiber-fiber-folded spectrometer. At present, the abundance analysis of Si is generally carried out under the assumption of local thermodynamic equilibrium (LTE) effect, but for those stars whose atmosphere deviates from the LTE state obviously, The effect of nonlocal thermodynamic equilibrium (NLTE) effect should be taken into account in abundance analysis. Our work is to obtain the spectra of 166 dwarf stars near the sun and to establish a stellar atmospheric model by means of spectral synthesis method. Under the assumption of local thermodynamic equilibrium and nonlocal thermodynamic equilibrium, the Si abundance of the sample stars is obtained. The aim of this paper is to study the effect of NLTE effect on the determination of Si abundance. We use kinematics and abundance criterion ([Mg/Fe]) to divide the star composition of sample stars, and study the evolution trend of [Si/Fe] in different star groups such as thin disk, thick disk and halo star. The first part of the thesis is the introduction, mainly introduces the present situation of the observation of the abundance of Si in the star, the second part gives the observation sample and the data processing process, the third part is the determination of the atmospheric model and the star parameter. Part 4th is the atomic model of Si and the influence of nonlocal thermodynamic equilibrium; part 5th gives the results of Si abundance and analysis of the results; part 6th is the discussion of the results; and the last part 7th is the conclusion. Through the analysis, we get the following important conclusions:. The effect of NLTE on Si abundance is related to the effective temperature of stars, and the correction of NLTE of hot stars is larger. The effect of NLTE effect on the abundance of Si elements in different ionization states is different, and the correction of Si I lines of neutral atoms can be neglected. However, the average abundance correction of primary ionization Si II lines is -0.06dex. this makes the abundance difference between Si I line and Si II line decrease from 0.07dex under LTE to 0.02dex. which indicates that it is necessary to consider NLTE correction. (3) the abundance of Si in the poor metal stars tends to decrease with the increase of [Fe/H], but the [Si/Fe] of the metal-rich stars increases slightly with the increase of [Fe/H]. The [Si/Fe] of the thick disk star is obviously higher than that of the thin disk star, but there is no strict distinction between the thin disk and the thick disk. There are some stars whose kinematic properties belong to the thin disk and the element abundance belongs to the thick disk. There are also some stars whose kinematic properties are thick disk and element abundance is thin disk. (5) although both Si and mg are typical elements, they were the product of massive SNe II supernova eruptions in the early stages of galaxy evolution. However, in the late stage of galactic evolution, Si and mg may have different nuclear synthesis mechanisms, and Si abundance may be partly due to the contribution of SNe Ia and AGB stars.
【学位授予单位】：河北师范大学
【学位级别】：硕士
【学位授予年份】：2012
【分类号】：P156

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