恒星Ba元素及其同位素丰度的NLTE分析

发布时间：2018-05-15 04:41

本文选题：银河系 + 星族　；参考：《河北师范大学》2013年硕士论文

【摘要】：恒星元素丰度代表恒星大气的化学组成，直接反映了恒星形成时所处的星系环境，是我们从观测上追踪恒星形成和星系化学演化的有效探针。由于不同元素的核合成过程和产生场所的演化时标不同，恒星中不同元素的丰度比在一定程度上反映了恒星形成和元素核合成的历史，，可作为星系形成和演化的宇宙时钟。特别是由于Ba元素的奇同位素(~(135)Ba和~(137)Ba)主要是r-过程核合成的产物，而偶同位素（~(134)Ba、~(136)Ba和~(138)Ba）主要是s-过程核合成的产物，因此其奇、偶同位素的丰度之比反映了r-过程~(和s-过程核合成对Ba丰度的相对贡献，由此可以更好地追踪r-过程和s-过程的核合成历史，研究其可能的产生场所，为星系化学演化模型和元素核合成理论提供可靠的观测约束。我们分别用局部热动平衡（LTE）和非局部热动平衡（NLTE）模型分析了28颗恒星的Ba元素丰度，金属丰度范围为-2.41[Fe/H]+0.07，包括16颗薄盘星、3颗厚盘星、7颗晕星，以及2颗运动学特征介于厚盘和薄盘之间的过渡恒星。并利用BaII共振线455.4nm的超精细结构和同位素结构分裂，确定了不同奇、偶同位素比例的Ba元素丰度，以此来研究不同星族恒星中r-过程和s-过程核合成对Ba元素丰度的相对贡献。论文内容包括6部分：(1)引言部分简单介绍了元素核合成理论和元素丰度概念，综述了对恒星Ba元素丰度的观测研究；（2）观测样本和数据处理，样本星的光谱数据取自欧洲南方天文台（ESO）甚大望远镜（VLT）及其阶梯光栅摄谱仪UVES公开释放的一维谱，光谱分辨率R40000，光谱信噪比S/N200。一维谱的视向速度修正和归一化是利用在IDL语言环境下开发的SIU程序包进行的；（3）确定恒星大气参数，包括有效温度T_(eff)、表面重力加速度logg、金属丰度[Fe/H]和微观湍流速度ξ，并建立恒星大气模型，包括局部热动平衡（LTE）和NLTE模型；（4）建立Ba原子模型，模型中考虑了Ba原子谱线的超精细结构和同位素结构分裂，包括BaII的35个束缚能级和BaIII基态能级；（5）Ba元素丰度分析和结果讨论：利用光谱综合方法，通过拟合观测到的BaII原子的3条谱线455.4nm、585.3nm和649.6nm的谱线轮廓，确定了样本星在LTE和NLTE模型下的Ba元素丰度，并且利用共振线455.4nm分析了几种不同奇、偶同位素比例（偶:奇=82:18,70:30,65:35,54:46）的Ba同位素丰度；根据丰度分析结果，讨论了不同星族包括晕星、厚盘恒星和薄盘恒星的[Ba/Fe]～[Fe/H]关系，以及不同星族恒星中r-过程和s-过程核合成对Ba元素丰度的相对贡献，分析了Ba丰度NLTE效应；（6）最后部分是论文的结论。通过对28颗恒星的Ba元素丰度的分析和讨论，我们得到以下重要结论： 1、不同谱线的NLTE效应不同：次级谱线的NLTE丰度修正[Ba/Fe]_(NLTE-LTE)都是负的且与恒星金属丰度无关，其中585.3nm的NLTE效应较弱，其丰度修正[Ba/Fe]_(NLTE-LTE)平均为-0.03dex；649.6nm的NLTE效应较强，其丰度修正[Ba/Fe]NLTE-LTE平均为-0.1dex，最大为-0.18dex。共振线BaII455.4nm的NLTE效应也比较明显，但其对丰度的修正依赖于恒星的金属丰度，当[Fe/H]-1.8时，[Ba/Fe]_(NLTE-LTE)也是负的，平均为-0.1dex；而当[Fe/H]-1.8时，[Ba/Fe]_(NLTE-LTE)转变为正的，平均在0.1dex。 2、BaII谱线的NLTE效应依赖于恒星的有效温度，无论是共振线还是次级谱线，其丰度修正[Ba/Fe]_(NLTE-LTE)都有随有效温度的升高而变大的趋势。尤其是次级谱线649.6nm的NLTE修正与有效温度的相关性较强，当恒星的Teff从5600K升高到6600K时，其[Ba/Fe]_(NLTE-LTE)从-0.04dex增加到-0.15dex。 3、在样本的整个金属丰度范围(-2.41[Fe/H]+0.07)内恒星的[Ba/Fe]丰度并不依赖于恒星的金属丰度且在平均值[Ba/Fe]=0附近变化，但却与恒星的星族成分有密切的关系。薄盘恒星的[Ba/Fe]与太阳丰度一致，平均为0.0±0.02dex，而厚盘恒星的[Ba/Fe]明显低于薄盘，平均为-0.1±0.02dex；运动学性质处于厚盘和薄盘之间的过渡恒星，其[Ba/Fe]的丰度与厚盘一致；晕星恒星的[Ba/Fe]弥散较大，随金属丰度的变化比较复杂，[Fe/H]-1.0时晕星的[Ba/Fe]丰度值在0附近，而当[Fe/H]-1.0时[Ba/Fe]随[Fe/H]的变低而由超丰变为欠丰，当[Fe/H]-2.0时[Ba/Fe]有快速下降趋势。 4、Ba的同位素丰度随金属丰度的变化与元素丰度[Ba/Fe]一致，而且也表现较强出的星族效应，即不同星族恒星中偶-奇同位素对Ba元素丰度的贡献比例不同。薄盘星中Ba元素偶-奇同位素的比并不是单一值，而是在82:18、70:30和65:35之间变化，表明薄盘恒星的Ba元素核合成过程比较复杂，多数薄盘恒星与太阳的Ba丰度一致且其奇偶核素比例也与太阳系混合比例相同，即s-过程的贡献为主，但有些薄盘恒星的Ba丰度主要是r-过程的贡献；对于厚盘星，Ba元素偶-奇同位素的比也不是单一的确定值，而是在65:35和54:46之间，这表明厚盘恒星的Ba元素核合成过程为r-过程占主要，低金属丰度的厚盘恒星的Ba几乎是纯r-过程的产物，但s-过程的贡献不能忽略；对于晕星，Ba元素偶-奇同位素之比为太阳系纯r-过程54:46，表明晕星的Ba元素来自于纯r-过程核合成，而且这与晕星的金属丰度无关。
[Abstract]:The abundance ratio of different elements in a star reflects the relative contribution of the formation of stars and the evolution of the elements . The ratio of the abundance ratios of different elements in the stars reflects the relative contribution of the r - process and the s - process nuclear synthesis to Ba abundance .

The Ba element abundance of 28 stars is analyzed by means of local thermal equilibrium ( LTE ) and non - local thermal equilibrium ( NLTE ) models . The range of metal abundance is - 2.41 掳 Fe / H ~ + 0.07 , including 16 thin plate stars , 3 thick disk stars , 7 halo stars , and 2 kinematic features between thick disk and thin disc .

The contents of this paper include 6 parts : ( 1 ) Introduction section simply introduces the theory of element nuclear synthesis and the concept of element abundance , and summarizes the observation and research on the abundance of Ba elements of stars ;
( 2 ) The spectral data of the sample star is taken from the one - dimensional spectrum , the spectral resolution R40,000 , the spectral signal - to - noise ratio S / N200 , which is released by the European Southern Observatory ( VLT ) and its step - grating spectrograph UVES , and the spectral data of the sample star is carried out by using the SIU package developed under the IDL language environment ;
( 3 ) determining stellar atmosphere parameters , including effective temperature T _ ( eff ) , surface gravity acceleration , metal abundance , Fe / H and micro turbulence velocity , and establishing stellar atmosphere model , including local thermal dynamic balance ( LTE ) and NLTE model ;
( 4 ) Ba atom model is established , and the hyperfine structure and isotope structure splitting of Ba atomic spectrum line are considered in the model , including 35 binding energy level and BaIII ground state energy level of BaII ;
( 5 ) The Ba element abundance analysis and the results are discussed . The Ba element abundance in the LTE and NLTE models is determined by fitting the spectral lines of 450.4nm , 585.3 nm and 649.6 nm of the observed BaII atom , and the Ba isotope abundance of several odd and even isotope ratios ( even : odd = 82 : 18 , 70 : 30 , 65 : 35 , 54 : 46 ) is analyzed by using the resonance line 450.4nm .
Based on the results of abundance analysis , the relative contributions of different stars including halo stars , thick disk stars and thin disk stars , and the relative contribution of r - process and s - process nuclear synthesis on the abundance of Ba elements are discussed , and the effects of Ba abundance NLTE are analyzed .
( 6 ) The last part is the conclusion of the paper .

Through analysis and discussion of the abundance of Ba elements of 28 stars , we have the following important conclusions :

1 . The NLTE effect of different spectral lines is different : the NLTE abundance modification of the secondary spectrum line is negative and independent of the abundance of the star metal , of which the NLTE effect of 585.3 nm is weak , and the abundance correction of the NLTE - LTE is - 0.03dex ;
The NLTE effect of 649.6 nm is stronger than that of NLTE - LTE - LTE with an average of - 0.1dex , and the maximum is - 0.18dex . The NLTE effect of the resonance line BaII450.4nm is also obvious , but its correction to abundance depends on the metal abundance of the star . When Fe / H reaches - 1.8 , the NLTE - LTE is also negative , and the average is - 0.1dex ;
However , when Fe / H alloy - 1.8 was added , Ba / Fe alloy _ ( NLTE - LTE ) was changed to positive , and the average was 0.1dex .

2 . The NLTE effect of the BaII spectral line depends on the effective temperature of the star , whether the resonant line or the secondary spectrum line , the abundance correction of the NLTE - LTE has a tendency to increase with the increase of the effective temperature . Especially the NLTE correction at 649.6 nm of the secondary spectrum line has a strong correlation with the effective temperature , and when the Teff of the star rises from 5600K to 6600K , it increases from - 0.04 dex to - 0.15dex when Teff of the star rises from 5600K to 6600K .

3 . The abundance of the stars in the whole metal abundance range of the sample ( - 2.41 掳 Fe / H ~ + 0.07 ) does not depend on the metal abundance of the star and varies around the average value of Ba / Fe isotope = 0 , but it is closely related to the star component of the star . The stellar Ba / Fe isotope of the thin disc star is consistent with the solar abundance , the average is 0.0 卤 0.02dex , while the thick disc star has a significantly lower Ba / Fe isotope than the thin plate , with an average of - 0.1 卤 0.02dex ;
The kinematic property is a transition star between thick disk and thin plate , and its abundance is consistent with thick disc .
When Fe / H becomes - 1.0 , Ba / Fe alloy has a higher value of Ba / Fe alloy . When Fe / H becomes - 1.0 , Ba / Fe alloys change from ultra - feng to underabundance . When Fe / H becomes - 2.0 , Ba / Fe alloy has a tendency to decrease rapidly .

The ratio of Ba element even - odd isotopes in thin plate stars is not a single value , but also varies between 82 : 18 , 70 : 30 and 65 : 35 , indicating that the Ba element nuclear synthesis process of thin disk stars is complex , and that the ratio of Ba element even - odd isotopes in thin plate stars is the same as that of the solar system , that is , the contribution of the s - process is dominant , but the Ba abundance of some thin disk stars is mainly the contribution of r - process ;
For thick - disc stars , the ratio of Ba element even - odd isotopes is not a single definite value , but between 65 : 35 and 54 : 46 , which indicates that the Ba element nucleus synthesis process of thick disc stars is r - process , Ba of thick disc stars with low metal abundance is almost the product of pure r - process , but the contribution of s - process cannot be ignored ;
For halo , the ratio of the Ba element to the odd isotope is the pure r - process 54 : 46 of the solar system , indicating that the Ba element of the halo is derived from the pure r - process nuclear synthesis , and this is independent of the metal abundance of the halo .

【学位授予单位】：河北师范大学
【学位级别】：硕士
【学位授予年份】：2013
【分类号】：P144

【参考文献】