本文關鍵詞： 銀河系 暈星 中子俘獲元素 元素豐度 核合成　出處：《河北師范大學》2017年碩士論文　論文類型：學位論文

【摘要】：恒星的元素豐度及其特征是我們追蹤恒星形成和化學演化的有效探針。Ba元素和Eu元分別是慢中子俘獲過程(s-過程)和快中子俘獲過程(r-過程)典型的代表元素,其豐度比[Eu/Ba]可作為研究恒星元素核合成過程的一個很好的示蹤器。另外,Ba元素的奇同位素(~(135)Ba和~(137)Ba)主要由r-過程產生,而偶同位素(~(134)Ba、~(136)Ba和~(138)Ba)主要由s-過程產生,因此,其奇、偶同位素的相對比例一定程度上代表了r-過程和s-過程對Ba豐度的相對貢獻。由此,我們可以根據Ba的這一元素特征追蹤r-過程和s-過程核合成歷史,研究其可能的產生場所,進而研究恒星的形成演化和起源。我們基于MAFAGS不透明度采樣恒星大氣模型,分別在局部熱動平衡(LTE)和非局部熱動平衡(NLTE)下,采用光譜綜合法分析了40顆暈星(包括13顆低α暈星和27顆高α暈星)的Ba和Eu的元素豐度,觀測光譜分別取自歐南臺(ESO)VLT/UVES釋放的光譜數(shù)據和北歐光學望遠鏡(NOT)及其光纖階梯光柵攝譜儀(FIES)的光譜數(shù)據。并且考慮到奇偶同位素對BaⅡ共振線λ=4554?超精細結構的影響,通過擬合該譜線的輪廓利用χ~2最小方法確定了Ba元素奇偶同位素的最佳比例,據此來推算r-過程和s-過程對Ba元素豐度的相對貢獻。我們的分析結果表明:(1)Ba元素豐度存在一定的NLTE效應,NLTE的豐度修正平均為-0.06dex,且基本與金屬豐度無關,但不同的譜線的NLTE效應是明顯不同的,其中λ6496?的NLTE效應最強,其豐度修正值平均為-0.15dex;其次是λ5853?,其NLTE豐度修正值平均為-0.03dex;λ6141?受到的NLTE效應最弱,其豐度修正值平均為-0.002dex。(2)Eu元素豐度存在明顯的NLTE效應,對不同金屬豐度的恒星其NLTE豐度修正最小為0.03dex,最大到0.12dex。(3)高α和低α暈星的[Ba/Fe]豐度不能明顯區(qū)分開,但[Eu/Fe]豐度卻表現(xiàn)出了明顯的區(qū)分,低α暈星的[Eu/Fe]反而高于高α暈星,并且在NLTE下這種區(qū)分度更加明顯。(4)低α暈星的Ba豐度大部分是r-過程和s-過程共同的貢獻,但總體上r-過程的貢獻比例更大,有的甚至是純r-過程的產物。而高α暈星的Ba豐度的核合成機制則更為復雜,既有來自純r-過程的貢獻,也有幾乎是純s-過程的貢獻,當然更多的來自s-過程和r-過程的共同貢獻,但二者的相對貢獻相對低α暈星來說更為不確定。綜合上述結果,我們認為高α和低α暈星可能有不同的形成場所,但是二者的產生場所都并不單一,高α暈星可能既有“本地形成”(In situ)也有來自年老厚盤“被踢出”(Kicked-out)的形成機制,而低α暈星也不僅僅來自于矮星系的“吸積”(Accreted),很可能是三種起源的某種混合,甚至還可能有其他的來源。
[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 偽 halos and 27 high 偽 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 偽 and low 偽 halo stars can not be distinguished obviously. However, the abundance of [Eu/Fe] shows a distinct distinction. The [Eu/Fe] of low 偽 halo stars is higher than that of high 偽 halo stars, and in NLTE, Ba abundance of low 偽 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 偽 -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 偽 and low 偽 halos may have different sites for formation. But there is not a single place to produce both, and the formation mechanism of high 偽 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.
【學位授予單位】：河北師范大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：P148

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