高低α晕族恒星的钡铕元素丰度及其核合成过程分析

发布时间：2018-02-13 20:58

本文关键词： 银河系晕星中子俘获元素元素丰度核合成　出处：《河北师范大学》2017年硕士论文　论文类型：学位论文

【摘要】：恒星的元素丰度及其特征是我们追踪恒星形成和化学演化的有效探针。Ba元素和Eu元分别是慢中子俘获过程(s-过程)和快中子俘获过程(r-过程)典型的代表元素,其丰度比[Eu/Ba]可作为研究恒星元素核合成过程的一个很好的示踪器。另外,Ba元素的奇同位素(~(135)Ba和~(137)Ba)主要由r-过程产生,而偶同位素(~(134)Ba、~(136)Ba和~(138)Ba)主要由s-过程产生,因此,其奇、偶同位素的相对比例一定程度上代表了r-过程和s-过程对Ba丰度的相对贡献。由此,我们可以根据Ba的这一元素特征追踪r-过程和s-过程核合成历史,研究其可能的产生场所,进而研究恒星的形成演化和起源。我们基于MAFAGS不透明度采样恒星大气模型,分别在局部热动平衡(LTE)和非局部热动平衡(NLTE)下,采用光谱综合法分析了40颗晕星(包括13颗低α晕星和27颗高α晕星)的Ba和Eu的元素丰度,观测光谱分别取自欧南台(ESO)VLT/UVES释放的光谱数据和北欧光学望远镜(NOT)及其光纤阶梯光栅摄谱仪(FIES)的光谱数据。并且考虑到奇偶同位素对BaⅡ共振线λ=4554?超精细结构的影响,通过拟合该谱线的轮廓利用χ~2最小方法确定了Ba元素奇偶同位素的最佳比例,据此来推算r-过程和s-过程对Ba元素丰度的相对贡献。我们的分析结果表明:(1)Ba元素丰度存在一定的NLTE效应,NLTE的丰度修正平均为-0.06dex,且基本与金属丰度无关,但不同的谱线的NLTE效应是明显不同的,其中λ6496?的NLTE效应最强,其丰度修正值平均为-0.15dex;其次是λ5853?,其NLTE丰度修正值平均为-0.03dex;λ6141?受到的NLTE效应最弱,其丰度修正值平均为-0.002dex。(2)Eu元素丰度存在明显的NLTE效应,对不同金属丰度的恒星其NLTE丰度修正最小为0.03dex,最大到0.12dex。(3)高α和低α晕星的[Ba/Fe]丰度不能明显区分开,但[Eu/Fe]丰度却表现出了明显的区分,低α晕星的[Eu/Fe]反而高于高α晕星,并且在NLTE下这种区分度更加明显。(4)低α晕星的Ba丰度大部分是r-过程和s-过程共同的贡献,但总体上r-过程的贡献比例更大,有的甚至是纯r-过程的产物。而高α晕星的Ba丰度的核合成机制则更为复杂,既有来自纯r-过程的贡献,也有几乎是纯s-过程的贡献,当然更多的来自s-过程和r-过程的共同贡献,但二者的相对贡献相对低α晕星来说更为不确定。综合上述结果,我们认为高α和低α晕星可能有不同的形成场所,但是二者的产生场所都并不单一,高α晕星可能既有“本地形成”(In situ)也有来自年老厚盘“被踢出”(Kicked-out)的形成机制,而低α晕星也不仅仅来自于矮星系的“吸积”(Accreted),很可能是三种起源的某种混合,甚至还可能有其他的来源。
[Abstract]:The element abundance of stars and their characteristics are typical representative elements of our effective probe. Ba and EU elements, which are the slow neutron capture process and the fast neutron capture process, respectively, which are effective probes for tracking star formation and chemical evolution. The abundance ratio [Eu/Ba] can be used as a good tracer for studying the nuclear synthesis process of stellar elements. In addition, the odd isotopes of Ba elements are mainly produced by r-process, and the odd isotopes of Ba elements are mainly produced by the r-process, while the occasionally isotopic elements 134-, 136Ba and 138Ba) are mainly produced by the s- process. The relative proportion of even isotopes to some extent represents the relative contribution of r-process and s-process to Ba abundance. Therefore, we can trace the history of r-process and s-process nuclear synthesis according to this element characteristic of Ba, and study its possible place of production. Then we study the formation, evolution and origin of stars. Based on the MAFAGS opacity sampling stellar atmosphere model, we have a local thermodynamic equilibrium (LTE) and a nonlocal thermodynamic equilibrium (NLTE), respectively. The elemental abundance of Ba and EU in 40 halos (including 13 low 伪 halos and 27 high 伪 halos) was analyzed by spectral synthesis method. The observed spectra were obtained from the spectral data released by ESOO VLT- UVES and from the Nordic Optical Telescope (NOT) and the fiber step grating spectrograph (FIES), respectively. By fitting the profile of the spectral line, the optimum proportion of the odd and even Ba isotopes is determined by 蠂 ~ (2) minimum method. The relative contribution of r-process and s-process to the abundance of Ba is calculated. The results show that there exists a certain NLTE effect on the abundance of Ba and the average correction of the abundance is -0.06 dexter, which is independent of the abundance of metal. But the NLTE effect of different spectral lines is obviously different, in which 位 6496? The NLTE effect was the strongest, and the average correction value of its abundance was -0.15 dexter, followed by 位 5853? The average NLTE abundance correction value is -0.03 dexter; 位 6141? The NLTE effect is the weakest, and the average correction value of its abundance is -0.002dex.Pu EU element abundance has obvious NLTE effect. For the stars with different metal abundance, the minimum correction of NLTE abundance is 0.03dexand, the maximum is 0.12dex.f3) the [Ba/Fe] abundance of high 伪 and low 伪 halo stars can not be distinguished obviously. However, the abundance of [Eu/Fe] shows a distinct distinction. The [Eu/Fe] of low 伪 halo stars is higher than that of high 伪 halo stars, and in NLTE, Ba abundance of low 伪 halo stars is mostly the contribution of r- process and s- process. But on the whole, the contribution of r-process is larger, some are even the product of pure r-process, and the nuclear synthesis mechanism of Ba abundance of high 伪 -halo star is more complicated, both from pure r-process and almost from pure S-process. Of course, there are more contributions from both s- processes and r-processes, but their relative contributions are more uncertain than the low-a-halos. Combined with the above results, we think that high 伪 and low 伪 halos may have different sites for formation. But there is not a single place to produce both, and the formation mechanism of high 伪 halos may be either "locally formed" or "kicked out" from the thick disk of old age, "Kicked-out". And the low alpha halo is not just from the accretion of dwarf galaxies, but probably from some sort of mixture of the three origins, and there may even be other sources.
【学位授予单位】：河北师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P148

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