平均中子辐照量计算公式对钡星的应用

最新的研究结果表明:目前流行的13C壳层(13C pocket)在热脉冲间隔期间辐射燃烧的低质量AGB(Asymptotic Giant Branch)星s-过程核合成模型,其核合成区域的中子辐照量分布可视为指数形式,平均中子辐照量τ0和模型参量之间的关系为τ0=-?τ/ln[q/(1-r+q)],式中?τ为每次照射的中子辐照量,r为重叠因子,q为13C壳层占氦中间壳层的质量比例.将该公式应用到26颗钡星样品星,对其可靠性进行了检验,在此基础上对26颗Ba星的中子辐照特点进行了分析.研究结果表明:公式是可靠的;26颗Ba星中,至少有8颗星的AGB伴星具有指数形式的中子辐照量分布,有12颗星极有可能经历指数中子辐照,有4颗星更倾向于经历单辐照;26颗Ba星的AGB伴星中,大多数在s-过程核素丰度达渐近分布前可能只经历很少次数的中子照射.     

计算平均中子辐照量的新公式

根据13C辐射燃烧的低质量AGB(Asymptotic Giant Branch)星s-过程核合成模型,考察了氦壳层核合成区域中子辐照量分布(以下简称DNE)的特点.结果表明,该模型的DNE同以前的对流核合成模型一样仍然非常接近指数分布,但是每个脉冲的中子辐照量△r和平均中子辐照量To之间的关系却不再是To=-△T/lnr,而近似为To=-△r/ln{q[1.0020 0.6602(r-q) 4.6125(r-q)2-10.8962(r-q)3 13.9138(r-q)4]}(r为重叠因子, q为13C壳层占氦壳层的质量比例).该式从DNE角度将辐射s-过程恒星模型和经典模型统一起来.     

AGB星辐射s-过程核合成模型中子辐照量分布的计算方法和特点

考虑到模型参量随脉冲数的变化,推导出13C辐射燃烧的低质量AGB星s-过程核合成模型中子辐照量分布的计算方法,该方法具有普适性和简洁性.利用该方法,计算了3 M☉、太阳金属丰度恒星模型的中子辐照量分布.结果表明,若合理假设13C薄层内中子数密度均匀分布,则辐射核合成模型最终的中子辐照量分布趋近于指数分布.对于初始质量一定、金属丰度一定的恒星模型,平均中子辐照量τ0与每个脉冲的中子辐照量△τ存在确定的关系:τ0=0.434λ(q1,q2…qmmax+1,r1,r2…rmmax+1)△τ,式中mmax是带挖掘的脉冲总数,比例系数λ(q1,q2…qmmax+1,r1,r2…rmmax+1)可通过对最终中子辐照量分布的指数拟合得到.该式从中子辐照量分布角度定量地将经典模型和辐射s-过程核合成模型统一起来,使经典模型能为恒星模型核合成数值计算提供指导和约束.     

平均中子辐照量计算公式对CEMP-s和CEMP-r/s星的应用

最近的研究结果表明,目前流行的~(13)C壳层(~(13)C pocket)在热脉冲间隔期间辐射燃烧的低质量AGB(Asymptotic Giant Branch)星s-过程核合成模型,其核合成区域的中子辐照量分布可视为指数形式,平均中子辐照量τ0和模型参量之间的关系为τ0=-?τ/ln[q/(1-r+q)],式中?τ为每次照射的中子辐照量,r为重叠因子,q为~(13)C壳层占氦中间壳层的质量比例.利用文献中参数化AGB星s-过程核合成模型对20颗CEMP(Carbon-Enhanced Metal-Poor)-s和CEMP-s/r星观测丰度的拟合结果,对该平均中子辐照量计算公式的可靠性进行了检验,并初步探讨其在重元素s-过程核合成理论研究中的作用.研究结果表明:在辐射s-过程核合成机制下,对于经历连续多次中子照射的CEMP星,公式是适用的;和参数化AGB星s-过程核合成模型结合,公式可以作为单辐照或特殊CEMP星的一个有效筛选工具.考虑到~(13)C壳层的不确定性,公式在理解CEMP星中子俘获元素观测丰度产生的物理条件方面的作用值得做进一步的探讨.     

AGB星s-过程核合成区域中子辐照量分布

介绍了AGB(Asymptotic Giant Brahch)星s-过程核合成区域中子辐照量分布的最新研究结果.然后将不同AGB星s-过程核合成模型给出的太阳系中子辐照量分布与由观测约束得出的结果进行对比,分析论证了最新研究结果的可靠性.     

极贫金属晕星中Eu元素的起源

快中子过程(r-过程)发生在富中子的环境中.但是它所在的具体天体物理环境并不是很清楚.随着观测的进展,近年发现了很多的极贫金属的晕星.它们有两个特征:一是快中子元素超丰,并且相对丰度与太阳上的一致.另一个特征是当金属丰度一样时,星体中快中子元素的含量有大的弥散性.这为研究r-过程起源提供了一种独特的途径.模拟计算的方法,用于研究星系演化的过程,以及相伴发生的恒星的快中子核素含量的弥散性,可用来了解快中子元素的起源这样得出的星系演化模型,不仅包含了气体区域自发的恒星形成,而且考虑了超新星爆发激发的恒星形成.结果显示,低质量端的超新星应是快中子核素的产生地.同时,超新星引起的星系演化的不均匀性不足以解释观测到的晕星快中子元素含量的弥散,因此这个问题还有待于更进一步研究.     

恒星内部核素~(56)Fe、~(56)Co、~(56)Ni和~(56)Mn电子俘获过程中微子能量损失高斯修正

采用高斯修正法,研究了核素~(56)Fe、~(56)Co、~(56)Ni和~(56)Mn电子俘获过程中微子能量损失.结果表明:对核素的Gamow-Teller(G-T)共振跃迁能级分布的高斯修正使中微子能量损失率增加.在低能跃迁电子俘获过程为主导地位的反应中,高斯修正对中微子能量损失的影响很小,而对高能G-T共振跃迁为主要的电子俘获过程的中微子能量损失的影响将大大增加.如核素~(56)Fe在密度ρ_7=100(ρ_7以10~7 mol·cm~(-3)为单位),高斯函数半宽度△=14.3,18.3,22.3 Mev时,修正差异大约达2个数量级,核素~(56)Ni在△=6.3,18.3Mev差异分别达60%和40%.     

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。     

中子辐照量指数分布函数式中比例系数和平均中子辐照量的计算公式

已有的研究表明,在AGB(Asymptotic Giant Branch)星s-过程核合成模型发展演化的3个典型阶段中,其核合成区域中子辐照量分布在有效取值范围内都可视为指数分布,即ρ_(AGB)(τ)=C/τ_0 exp(-τ/τ_0),但式中比例系数C和平均中子辐照量τ_0的具体表达式相关文献中并未全部给定.通过深入剖析中子辐照量指数分布函数的基本求解方法,并系统梳理不同恒星模型中子辐照量分布函数的求解过程,得到了C和τ_0的计算通式及其辅助关系式.只要确定了恒星模型中子辐照量的分立分布函数,就可以据此组公式确定出C和τ_0与模型参量之间的关系式.所得结果有效地解决了利用解析方法求解目前流行的~(13)C壳层(~(13)C pocket)辐射燃烧AGB星s-过程核合成模型中子辐照量分布问题.     

两颗CEMP-s视单星化学元素的天体物理来源研究

CS30301–015和HE1045+0226是两颗C元素和s-过程元素均超丰的贫金属(CEMP-s)星.视向速度观测发现这两颗星可能为单星.采用叠加与分解的方法探究这两颗星化学元素的天体物理来源能够为更好地理解银河系早期化学演化提供线索.计算结果表明:这两颗星的轻元素和Fe族元素主要产生于大质量星的primary过程.对于CS 30301–015,中子俘获元素主要来自AGB (Asymptotic Giant Branch)星中的主要s-过程. Pb的显著超丰主要归因于主要s-过程的贡献(约占Pb观测丰度的99.8%).需要更多的视向速度观测来确定这两颗星的轨道特征.对于HE 1045+0226, 56Z (质子数)62的重中子俘获元素主要来源于主要s-过程; Eu主要来源于主要r-过程.而轻中子俘获元素Y和Zr主要来自快速自转大质量星的primary弱s-过程,这一核合成过程对HE 1045+0226的Y和Zr丰度的贡献分别约为69.8%和67.6%.这从观测的角度证明弱s-过程能够在贫金属环境下发生.     

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.     

Radioactive Ages of Metal-Poor Halo Stars

The abundances of long-lived radioactive elements Th and U observed in metal-poor halo stars can be used as chronometers to determine the age of individual stars, and hence set a lower limit on the age of the Galaxy and hence of the universe. This radioactive dating requires the zero-decay productions of Th and U, which involves complicated r-process nucleosynthesis calculations. Several parametric r-process models have been used to calculate the initial abundance ratios of Th/Eu and U/Th, but, due to the sharp sensitivity of these models to nuclear physics inputs, the calculations have relatively large uncertainties which lead to large uncertainties in the age determinations. In order to reduce these uncertainties, we present a simple method to estimate the initial productions of Th and U, which only depends on the solar system abundances and the stellar abundances of stable r-process elements. From our calculations of the initial abundance ratios of Th/Eu and U/Th, we re-estimate the ages of those ver     

贫金属星中子俘获元素丰度

贫金属星的中子俘获元素丰度与恒星的形成和演化密切相关,它为研究星系形成早期的历史背景和化学演化提供了重要信息。贫金属星中子俘获元素丰度的研究已成为近年来核天体物理研究的前沿和热点。介绍了恒星内部重元素的核合图像,ｓ过程和ｒ过程核合成的概念及其核合成场所。着重介绍了近年来有关贫金属星中子俘获元素丰度的观测结果,综述了近年来贫金属星子俘获元素分布的理论研究进展情况和中子俘获元素的星系化学演化的研究进展     

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.     

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.     

Heavy-elements in metal-poor stars: an UV perspective

The site(s) of the r-process(es) is(are) not completely defined, and several models have been proposed. Observed abundances are the best clues to bring some light to this field, especially the study of the extremely metal-poor (EMP) Galactic halo stars. Many elements can be measured using ground-based facilities already available, but the ultraviolet window also presents a rich opportunity in terms of chemical abundances of heavy elements. In fact, for some elements only the UV transitions are strong enough to be useful. Focusing on the project of the Cassegrain U-Band Brazilian Spectrograph (CUBES), we discuss the science case for heavy elements in metal-poor stars, describing the useful lines of trans-Fe elements present in the UV region. Lines in the far UV are also discussed.     

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.     

Metallicities and ages of stellar populations at a high Galactic latitude field

We present an analysis of UBVRI data from the selected area SA 141. By applying recalibrated methods of measuring ultraviolet excess (UVX), we approximate abundances and absolute magnitudes for 368 stars over 1.3 deg² out to distances over 10 kpc. With the density distribution constrained from our previous photometric parallax investigations and with sufficient accounting for the metallicity bias in the UVX method, we are able to compare the vertical abundance distribution to those measured in previous studies. We find that the abundance distribution has an underlying uniform component consistent with previous spectroscopic results that posit a monometallic thick disc and halo with abundances of  [Fe/H]=−0.8  and −1.4, respectively. However, there are a number of outlying data points that may indicate contamination by more metal-rich halo streams. The absence of vertical abundance gradients in the Galactic stellar populations and the possible presence of interloping halo streams would be consistent with expectations from merger models of Galaxy formation. We find that our UVX method has limited sensitivity in exploring the metallicity distribution of the distant Galactic halo, owing to the poor constraint on the UBV properties of very metal-poor stars. The derivation of metallicities from broad-band UBV photometry remains fundamentally sound for the exploration of the halo but is in need of both improved calibration and superior data.     

New Non-Gaussian Feature in COBE-DMR 4 Year Maps

Recent determinations of the abundance of the light-element Li in very metal-poor stars show that its intrinsic dispersion is essentially zero and that the random error in the estimated mean Li abundance is negligible. However, a decreasing trend in the Li abundance toward lower metallicity indicates that the primordial abundance of Li can be inferred only after allowing for nucleosynthesis processes that must have been in operation in the early history of the Galaxy. We show that the observed Li versus Fe trend provides a strong discriminant between alternative models for Galactic chemical evolution of the light elements at early epochs. We critically assess current systematic uncertainties and determine the primordial Li abundance within new, much tighter limits: &parl0;Li&solm0;H&parr0;p=1.23+0.68-0.32x10-10. We show that the Li constraint on OmegaB is now limited as much by uncertainties in the nuclear cross sections used in big bang nucleosynthesis (BBN) calculations as by the observed abundance itself. A clearer understanding of systematics allows us to sharpen the comparison with 4He and deuterium and the resulting test of BBN.     

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.