贫金属星重元素丰度分布的参数化研究

基于大量贫金属星元素丰度的观测资料,以太阳系重元素丰度分布为标准,选取Sr、Ba、Eu分别作为贫金属星弱s-过程、主要s-过程、r-过程3种核合成的典型元素,采用参数化方法,分析了不同核合成过程对贫金属星重元素丰度的贡献比例.研究表明:金属丰度越高,弱s-过程、主要s-过程对较轻的中子俘获元素丰度的贡献就越大,较重的中子俘获元素主要由r-过程和主要s-过程产生;金属丰度较低,重中子俘获元素丰度主要由r-过程产生,星系早期弱s-过程对元素丰度几乎没有贡献.     

贫金属星中子俘获元素的丰度分布

在提出的贫金属星中子俘获元素丰度的计算模型基础上研究1999年新发表的21颗贫金属星的中子俘获元素丰度分布。结果表明，对较重的中子俘获元素理论预测曲线与观测值符合得很好，而对较轻的中子俘获元素二者有所偏离。这表明在贫金属环境下，对较重的中子俘获元素各核合成过程产生的丰度分布与太阳系中相应过程的丰度分布相似，但贡献比例与太阳系不同；而对较轻的中子俘获元素丰度分布与太阳系的丰度分布有所偏离；这也说明较轻的和较重的中子俘获元素的核合成场所不同，即具有不同的核合成机制。同时还特别讨论了丰度观测误差对表征各核合成过程的分量系数的影响。     

CEMP星HE 1005-1439中子俘获元素天体物理来源的研究

HE1005-1439是一颗金属丰度极低([Fe/H]～-3.0)的碳增丰贫金属星(Carbon Enhanced Metal-Poor,CEMP),该星的s-过程元素显著超丰([Ba/Fe]=1.16±0.31,[Pb/Fe]=1.98±0.19),而r-过程元素温和超丰([Eu/Fe]=0.46±0.22),使用单一的s-过程模型和i-过程模型均不能拟合该星中子俘获丰度分布.采用丰度分解的方法探究该星化学元素的天体物理来源可有助于理解CEMP星的形成和化学演化.利用s-过程和r-过程的混合模型对其中子俘获元素的丰度分布进行拟合,发现该星的中子俘获元素主要来源于低质量低金属丰度AGB伴星的s-过程核合成,而r-过程核合成也有贡献.     

一种研究贫金属星重元素丰度分布的方法

以重元素核合成理论及星系化学演化理论所得结论为基础,提出了一种利用３ 种中子俘获过程的典型元素丰度观测值,研究在不同金属度下不同核合成过程对重元素丰度的贡献的方法．由此可以确定ｒ过程与ｓ过程的贡献比例,并可用此方法来计算其余重元素丰度,得到关于重元素核合成的重要信息．利用这种方法计算了贫金属星ＨＤ６２６４４ 、ＨＤ１２６２３８和ＨＤ１８４７１１ 的重元素丰度,并将计算结果与观测结果进行了比较、分析和讨论了计算结果的物理意义     

CEMP星HE1005-1439中子俘获元素天体物理来源的研究

HE1005-1439是一颗金属丰度极低([Fe/H] ~ - 3.0)的碳增丰贫金属星(Carbon Enhanced Metal-Poor,CEMP), 该星的s-过程元素显著超丰([Ba/Fe] = 1.16±0.31, [Pb/Fe] = 1.98±0.19), 而r-过程元素温和超丰([Eu/Fe] = 0.46±0.22), 使用单一的s-过程模型和i-过程模型均不能拟合该星中子俘获丰度分布. 采用丰度分解的方法探究该星化学元素的天体物理来源可有助于理解CEMP星的形成和化学演化. 利用s-过程和r-过程的混合模型对其中子俘获元素的丰度分布进行拟合, 发现该星的中子俘获元素主要来源于低质量低金属丰度AGB伴星的s-过程核合成, 而r-过程核合成也有贡献.     

碳超丰贫金属拐点星的搜寻与丰度分析

碳超丰贫金属星在老年恒星中的普遍存在使得它成为研究宇宙早期形成和演化的重要对象,具有不同中子俘获元素丰度特征的碳超丰贫金属星有不同的起源。不同于已经历挖掘过程的巨星,主序拐点星能很好地在其表层大气保留形成初期的物质,从而可以追溯到更早期的恒星形成化学环境。通过LAMOST巡天数据选源并利用Subaru/HDS进行后续高分辨率光谱观测了12颗碳超丰贫金属星主序拐点星,其中有10颗星是首次被分析。与已有的碳超丰贫金属星主序拐点星观测样本相比,文中的样本包含了更高比例的中子俘获元素不超丰碳超丰贫金属恒星(CEMP-no星),发生在中等程度碳超丰区域,为其起源研究提供了重要数据。对其中6颗CEMP-no星测定了锂丰度,尤其是其中3颗极贫金属星样本,为探讨[Fe/H]-3.0的碳超丰贫金属星拐点星锂丰度特征提供了重要的新观测数据。新增样本验证了CEMP-no星在[Fe/H]～-3.0附近为锂-正常星,与前人结论一致;而新增的两颗CEMP-s星表现出了中等锂丰度,介于之前的较大锂丰度范围,研究为CEMP-s和CEMP-no的分类提供了辅助约束条件。     

化学元素的起源问题

本文对化学元素起源问题的研究现状作了综合性的评述。一部分是元素的恒星合成理论,包括元素通过核聚变、轻核俘获反应及中子俘获而产生的机制,以及用星系演化讨论元素丰度的有关问题。另一部分是轻元素的宇宙学合成理论,包括早期宇宙中轻核形成机制,丰度的计算结果,原始丰度的观测确定,有关的争议以及支持宇宙学核合成理论的论据。     

恒星锂丰度研究进展

锂是少数几种在大中生成的元素之一,研究锂丰度对于探讨各种元素核合成理论以及星系的早期化学演化规律都具有十分重要的意义,阐述了有关恒星（类太阳星,晕族恒星和主序前得）及星团锂丰度的新近观测结果。介绍了在锂的核合成理论研究方面非局部热动平衡效应的影响及锂在恒星演化中的衰竭机制等理论的研究进展和存在的问题。     

晚型星的锂丰度与活动性

具有类太阳活动恒星的化学元素丰度观测与分析，对于研究星系的化学演化规律和恒星核合成过程都具有十分重要的意义。本文从观测和理论模型两个方面详细地评述了近年来国内外对类太阳活动恒星，特别是ＲＳＣＶｎ型星的锂丰度反常与活动性及自转速度关系的研究进展情况。     

Mg超丰恒星的性质及其起源的研究

Mg超丰恒星([Mg/Fe]1.0)的特殊丰度模式无法用普通恒星的Mg元素起源和银河系化学演化机制解释。对这类特殊天体的起源和演化及化学丰度性质的研究,有助于深化理解恒星核合成及星系演化中一些特殊过程。首先介绍了目前文献中由高分辨率光谱证认的Mg超丰恒星,并对这些恒星的大气参数、运动学参数和化学丰度特征等性质及其起源机制进行了分析。其次统计了在斯隆巡天数据中系统搜寻的Mg超丰恒星候选体的大气参数和运动学分布特征,并且筛选了其中C超丰的候选体。研究发现绝大部分Mg超丰恒星表现了C超丰;在Mg超丰恒星中,存在中子俘获元素超丰的那些恒星都存在于双星系统中,其演化过程受到了AGB伴星的影响;而没有表现中子俘获元素超丰的那些恒星极有可能起源于第一代低能量超新星,部分恒星具有很高的空间速度,这类空间速度大于300 km/s的Mg超丰恒星可能是搜寻第一代恒星([Fe/H]-5.0)的最好样本。     

A Study on the Abundance Distribution of Heavy Elements in Metal-poor Stars by the Parameterization Method

Based on a large amount of observed data of element abundances in metal-poor stars, taking the abundance distribution of heavy elements in the solar system as a standard, and selecting Sr, Ba and Eu as the typical elements of the three nucleosynthetic processes in metal-poor stars, namely the weak sprocess, main s-process and r-process, we have studied the contributions of the three kinds of neutron-capture processes to the abundance distribution of heavy elements in metal-poor stars, with the parameterization method. It is found that the higher the metal abundance, the greater the contributions of the weak s-process and the chief s-process to the abundances of lighter neutron-capture elements. The heavier neutron-capture elements are mainly produced by the r-process and the chief s-process; and that at low metallicity, the abundances of heavy neutron-capture elements are mainly produced by the r-process. In the early Galaxy, the weak s-process has almost no contribution to the element abundance.     

贫金属星r过程核合成相关问题的研究进展

快中子俘获过程(r过程)可以解释大约一半比铁重的稳定(和一些长寿命放射性的)富中子核素的产生,这已经被太阳系及各种金属丰度下恒星的观测结果所证实.为建立r过程模型,需要大量的核物理信息:涉及到β稳定谷与中子滴线之间的各种核素的稳定特性及β衰变分支等物理参数,实验和理论都面临巨大的挑战.综述了近年来贫金属星r过程核合成理...     

Two R-Process Components in Ultra-Metal-Poor Stars: The Neutron-Capture Element Distribution of CS 22892-052

The abundance patterns of neutron-capture elements in very metal-poor halo stars play a crucial role in guiding and constraining theoretical models of nucleosynthesis. Many studies have suggested that the abundance patterns of the heavier (Z≥ 56) stable neutron-capture elements in very metal-poor halo stars are consistent with the solar system r-process abundance distribution, but this concordance breaks down for the lighter neutron-capture elements in the range of 40<Z<56. Some studies argue that there are two separate r-processes respectively responsible for the productions of the heavier and lighter neutron-capture elements. The new observed data of the lighter n-capture elements in the 40<Z<56 domain (Nb, Ru, Rh, Pd, Ag and Cd) in CS 22892-052 makes it available to examine whether or not there are two different r-processes. Based upon these observed abundances of n-capture elements in ultra metal-poor star CS22892-052, we present a phenomenological model to identify the characters of the different nucleosynthesis processes in very metal-poor stars. The results show that the model predictions can well match the observations in CS 22892-052, which truly means that there are different r-processes for the lighter and heavier neutron-capture elements, and the stellarr-process patterns are similar to the solar system r-process abundance distribution. This revised version was published online in July 2006 with corrections to the Cover Date.     

AGB星s-过程核合成相关问题的研究进展

综述了近年来AGB星核合成理论的研究情况,述及AGB星的结构与s-过程核合成有关的中子辐照量分布、人们比较关注的铅星与非铅星、后AGB星元素丰度分布及与AGB星核合成有关的s r星。     

Abundance Ratios in Extreme Metal-Poor Stars

In extremely metal-poor stars ([Fe/H]≤ − 2.5) the neutron capture elementsare characterized by a 300-fold dispersion in M/Fe ratios which decreases with increasing metallicity, the median M/Fe ratio increases with increasing [Fe/H], but the averageM/Fe number ratio is approximately constant. These observations are consistent witha highly dispersed intrinsic yield of neutron-capture elements in supernova (SN) events,and a progression to increasing metallicity by stochastic chemical evolution.The abundance trends indicate that the synthesis of elements heavier thanbarium was dominated by the r-process. The Sr/Ba ratio shows a dispersionwhich suggests a stochastic source of Sr in excess of the r-process value;possibly due to the alpha-rich freeze out.The iron-peak elements Cr, Mn, and Co show non-solar abundance ratios forextreme metal-poor stars, and no measurableintrinsic dispersion relative to iron. We discuss chemical evolution models which explain these observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

贫金属富碳天琴RR变星的形成

贫金属富碳恒星(Carbon-Enhanced Metal-Poor, CEMP)是研究宇宙早期恒星性质和化学演化的极佳样本,通常认为来自双星.目前发现的贫金属富碳星中有9颗天琴RR变星(RR Lyrae star, RRL),其中至少7颗未表现出任何双星特征.传统双星物质转移模型不足以充分解释贫金属富碳天琴RR变星(CEMP-RR Lyrae)单星的形成.之前研究表明氦白矮星和赫氏空隙星(HG)的并合模型可以解释部分富碳红巨星单星的碳增丰现象,因此贫金属富碳星单星也可能来自氦白矮星和赫氏空隙星的并合模型渠道.通过详细计算的氦白矮星和赫氏空隙星并合模型来检验这一演化渠道,结果表明:该并合模型在后续的演化过程中,其重力加速度、温度、表面碳丰度均能与观测符合较好.由此,氦白矮星和赫氏空隙星并合模型极有可能是贫金属富碳天琴RR变星的形成渠道之一.     

Radio Emission from Three-dimensional Relativistic Hydrodynamic Jets: Observational Evidence of Jet Stratification

High-resolution, high signal-to-noise ratio, blue-violet spectra of three red giant branch tip stars in M15 have been obtained with the Keck I High-Resolution Echelle Spectrograph. These spectra have been analyzed to determine the abundances of several neutron-capture elements, including the radioactive chronometer element thorium. There are two principal results of this study. First, the abundances of the heavier (Z>/=56) elements for each of the three stars is well matched by a scaled solar system r-process abundance distribution. Second, a weighted mean-observed Th/Eu ratio for the stars implies an age for the neutron-capture material in M15 stars of 14+/-3 Gyr, in reasonable agreement with other recent age estimates for Galactic globular clusters.     

An explanation of the correlations of abundance ratio between neutron-capture elements and iron group elements in Ba stars

The chemical abundances of the Ba stars are excellent information for setting constraints on models of s-processes nucleosynthesis. In this work, we adopt a new analysis approach to determine the relative contributions from individual neutron-capture processes to the elemental abundances of Ba stars. We find that the production of s-process elements should accompany by the production of Cu and Zn, the calculated results on Cu and Zn abundances are in quite good agreement with observed data. The observed [Cu, Zn/Fe]–[s/Fe] correlations of Ba stars can be explained by binary scenario in which Ba stars formed.     

Final Stages of Stellar Evolution and Nucleosynthesis: What do We Know and What Would We Like to Know?

Recent developments in theoretical model-calculations for the synthesis of the chemical elements during late stages of stellar evolution are reviewed. Special emphasis is put on a discussion of various astrophysical sites, including core-collapse and thermonuclear supernovae, and the physics of turbulent reactive fluids. Results of numerical simulations are presented and discussed, together with new results concerning solar-system abundances as well as abundances observed in very metal-poor stars, in the context of searches for constraints on the still rather uncertain nuclear physics data and astrophysical models. This revised version was published online in September 2006 with corrections to the Cover Date.