r-过程元素产量、产区及星系化学演化

【作者】 陈哲；

【导师】 张波；

【作者基本信息】 河北师范大学 ， 理论物理， 2005， 硕士

【摘要】 本文共分两部分,第一部分介绍不同质量Ⅱ型超新星较重元素的r-过程核合成产量的计算并且确定r-过程核合成的主要场所,第二部分重点介绍了星系r-过程元素(从Ba到Eu)丰度的离散及星系化学演化。 本文给出在观测到的极贫金属星Ba元素随金属丰度分布区域的左边界与单颗Ⅱ型超新星污染区域的r-过程元素丰度对应的假定下,采用Tsujimoto给出的Ⅱ型超新星爆发Mg元素的产量,利用Tsujimoto提出的方法,根据观测到的极贫金属星Ba和Mg丰度的左边界数值计算各种质量超新星r-过程的产量,得出星系中r-过程核合成的主要质量区间。根据本文所得到的不同质量Ⅱ型超新星r-过程的产量关系,改进Fields等人所提出的方法,解释观测到的贫金属星中子俘获元素的弥散性(scatter);r-过程元素的均匀化学演化可以看成r-过程核合成场所的另一重要约束条件,本文还根据三成份(晕、厚盘和薄盘)多相模型(气体、分子云、大小质量恒星以及剩余物质),利用本文所得到的Ⅱ型超新星的r-过程的产量计算了r-过程元素的均匀化学演化,得到如下结论: (1)R-过程产量较高的Ⅱ型超新星质量区域约为20-40M_⊙。,这个质量区间所占Ⅱ型超新星总数的比例大约为18%,这虽然比Fields文中class A给出的比例略高,但从物理上与Fields所说的高r-过程产生场所相对应,而Fie1ds所说的低r-过程产生场所实际上是指上述质量区域以外的其它质量的Ⅱ型超新星,主要对应于低质量Ⅱ型超新星M<18M_⊙,约占82%。 (2)从星系化学演化角度看,r-过程核合成主要产生场所是较高质量的Ⅱ型超新星,质量范围在20M_⊙≤M≤25M_⊙,星系r-过程元素的主要来源为大质量星,r-过程核合成产量几乎不依赖于恒星的初始化学组成,即r-过程核合成区域内的种子核主要来自于恒星内部而不是初始丰度。 (3)关于星系重元素丰度的离散及星系r-过程元素丰度的均匀化学演化计算结果与贫金属星丰度观测结果基本符合说明本文关于极贫金属星重元素丰度分布的低金属边界与单颗Ⅱ型超新星的污染区域相对应的假设并由此得到的r-过程元素产量是较为合理的。 由此可以解释以下观测事实:更多还原

【Abstract】 The dissertation consists of two sections. The first one mainly introduces the r-process nucleosynthesis yields of heavy elements in SN II with different progenitor mass and dominating sites for r-process nucleosynthesis, and the second mainly introduces the scatter and the chemical evolution of Ba peak elements.In this letter, we assume that the left boundary to [Ba/Mg]-[Mg/H] plan has aone-to-one correspondence to the Ba yield from individual SN II; others correspond to the yields of several supernovae. So we use Mg yield and the same method of Tsujimoto, then obtain main mass range for r-process. According to obtained the function of r-process yield and SN II progenitor mass and improving the method of Fields , we can explain scatters of n-capture elements in metal poor stars and take advantage of the model named three-zone multiphase model in order to study the chemical evolution of the Galaxy . As described above the results of our model are as follows:（l）The mass sites of high r-process production are stars of 20-40M_☉. The ratio of the SN II number in this mass range to all SN II is 18%, that is higher than ratio of class A .（2）The mass range of SN II for the main r-process site is constrained to 20 - 25M_☉ .TheGalaxy r-process elements mainly come from massive stars, r-process nucleosynthesis yields nearly do not depend on initial chemical composition.（3）The assumption that the left boundary to [Ba/Mg]-[Mg/H] plan has a one-to-onecorrespondence to the Ba yield from individual SN II is reasonable for the scatters of heavy elements in halo stars and the results of chemical evolution in our paper are basicly accordance with observation.Therefore, many observed facts can be explained（l）Mg and Eu are both produced in massive stars and their yields increase with increasing progenitor masses .The high Eu yields of the massive stars deduce [Eu/Mg]>0, however ,the mass range of SN II that produces Mg is larger than the one that produce Eu , so many times of SN II explosion induce [Eu/Mg]<0.（2）Ba yield of progenitor mass 20 M_☉ is 3×10^-6M_☉ , which is accordance withobservation; using r-process elements yields and mFe ≈ 0.07M☉ ,we also obtain the value ofR ≈ 50 ,which is the same as Fields.（3）Mass range of SNII r-process is narrow , which maybe the reason why r-process abundance attributions of all metal poor stars are similar with that of the Sun, because physics conditions for r-process nucleosynthesis are similar.（4）From this essay, the calculations of element abundance scatters like Eu、 Ba、 Ce、 La、 Nd、 Pr、 Sm in the Galaxy halo reflect: the scatters in field stars abundances with [Fe/H]<-2.5 are very large, because the elements in these stars have been produced in a small number of nucleosynthesis events; with the increasing of the mental abundance, the scatters of heavy element abundances in various areas decrease gradually for the mixing of interstellar medium （ISM）, then the observation that the dispersion of the heavy element abundances in mental-poor stars becomes little with the increasing of the mental abundances can be explained.（5）In case B of Ba ,La,Ce and Nd, model results are lower than observation because s-process contribution（Travaglio et al.1999） dominates the Galaxy evolution of these elements starting from [Fe/H] - -2.0; the abundances of Eu,Sm and Pr in our model are accordance with the observation.Recently, the abundance evolution of n-capture elements in metal-poor stars plays more and more important and active role in chemical evolution of the Galaxy. In order to further study the true picture of heavy-element nucleosynthesis in metal-poor stars and the early history of the Galactic chemical evolution, more and more accurate observed data are needed to precisely determine the relative contributions of different n-capture processes to n-capture elements. Moreover, the data of lighter n-capture elements, especially the abundance of elements with mass number A around 100, are also required to explore the contribution of the weak s-process. With those improving, the research of neutron-capture elements abundance distribution and the chemical evolution of the Galaxies will come into a new stage, the results of which can be widely used in many respects of nuclear astrophysics.Certainly, in studying the history of the Galaxy chemical evolution （GCE） ,we need more accurate observed data. Our letter is just a elementary study in the field. There are still更多还原