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。     

中等贫金属星的元素丰度分析

给出１１颗中等贫金属星－１．３≤「Ｆｅ／Ｈ」≤－０．６的观测资料,确定了这些样本星的恒星大气参数,得到这些星中１４种元素的丰度,讨论了各类元素丰度比随金属经,α元素（Ｍｇ、Ｓｉ、Ｃａ、Ｔｉ）相对于铁均表现超丰,每一金属丰度处都存在不同程度的弥散。大部分的铁峰元素（Ｖ、Ｃｒ、Ｎｉ、Ｃｕ）也相对于铁超丰,不随「Ｆｅ／Ｈ」而变化,百「Ｍｎ／Ｆｅ」则随金属丰度的增加而增加。「Ｎａ／Ａｌ」在「Ｆｅ／Ｈ〉０－     

中等贫金属星的元素丰度分析

给出 １１颗中等贫金属星－１．３ ５［Ｆｅ／Ｈ］ ≤－０．６的观测资料,确定了这些样本星的恒星大气参数,得到了这些星中１４种元素的丰度．讨论了各类元素丰度比随金属丰度的变化。α元素（Ｍｇ、 Ｓｉ、 Ｃａ、 Ｔｉ）相对于铁均表现超丰,每一金属丰度处都存在不同程度的弥散．大部分的铁峰元素（Ｖ、Ｃｒ、Ｎｉ、Ｃｕ）也相对于铁超丰,不随［Ｆｅ／Ｈ］而变化,而［Ｍｎ／Ｆｅ］则随金属丰度的增加而增加．［Ｎａ／Ａｌ］在［Ｆｅ／Ｈ］＞－１．１的样本星中表现超车,在［Ｆｅ／Ｈ］＜－１．１的样本中却明显过贫．     

极贫金属星元素丰度及Ⅱ型超新星核合成产量

采用与Tsujimoto类似的方法,根据观测到的极贫金属星中各元素与Si丰度的相关性,利用WW95和FM2004给出的Ⅱ型超新星Si的理论产量,计算了极贫金属环境中各种质量超新星α-元素,铁族元素的产量,并将所得结果与前人的结果进行了比较。     

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.     

新前身星模型对超新星爆发的影响

采用2002年Woosley给出的前身星模型,使用"WZYWS9"程序数值模拟了质量为11-40M⊙的Ⅱ型超新星的爆发过程.计算结果显示新模型对Ⅱ型超新星塌缩、激波传播及爆发都有不同程度的影响.此外,还讨论了激波初始能量的定义.     

黑洞双星的形成

目前,观测证认的黑洞双星在黑洞质量分布上存在一峰值,但是还没有比较完备的理论可以解释,试图对该分布规律给出较合理的解释．鉴于恒星的氦(He)核在黑洞形成中所扮演角色的重要性,将He核质量与黑洞初始质量联系起来．根据Fryer等人的二维核塌缩模拟的结果,找到了一拟合关系,从而根据He核大小可以估算黑洞的初始质量．采用PPE恒星演化程序计算确定He核质量．最后得到星族Ⅱ和星族Ⅰ黑洞双星的黑洞初始质量的分布,并结合观测给出了相应的解释．     

贫金属星重元素平均丰度的金属丰度分区研究

将[Fe／H]从-3．0到0．0等分成10个区间,利用三种中子俘获过程的典型元素的观测丰度平均值,研究各核合成过程在各区间对重元素丰度的平均贡献,以及其它重元素的平均丰度,并与观测平均丰度进行比较．结果表明,在[Fe／H]＞－2．2范围内,各重元素核合成过程产生的重元素平均丰度分布与太阳系相似,但不同核合成过程的平均贡献比例与太阳系不相同．     

低部主序恒星锂和铍的衰减

根据化学非均匀恒星的非局部对流理论,计算了质量为0．7-1．15M(?)恒星主序演化模型锂和铍的衰减,并将理论和不同年龄疏散星团锂丰度的观测进行了比较．结果表明,贯穿对流混合能重现温度低于6400 K的晚型主序星锂丰度观测的一般性质．贯穿对流混合可能是晚型矮星锂衰减的一种重要机制．     

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     

Non－LTE Analysis of the Sodium Abundance of Metal－Poor Stars in the Galactic Disk and Halo

We performed an extensive non-LTE analysis of the neutral sodiumlines of Na I 5683/5688, 5890/5896, 6154/6161, and 8183/8195 in disk/halo starsof types F-K covering a wide metallicity range (-4 [Fe/H] +0.4), using ourown data as well as data collected from the literature. For comparatively metal-rich disk stars (-1 [Fe/H] +0.4) where the weaker 6154/6161 lines are thebest abundance indicators, we confirmed [Na/Fe] ～ 0 with an "upturn" (i.e., ashallow/broad dip around -0.5 [Fe/H] 0) as already reported in previousstudies. For the metal-deficient halo stars, where the much stronger 5890/5896 or8183/8195 lines subject to considerable (negative) non-LTE corrections amountingto 0.5 dex have to be used, our analysis suggests mildly "subsolar" [Na/Fe] valuesdown to ～ -0.4 (with a somewhat large scatter of ～±0.2 dex) on the average at thetypical halo metallicity of [Fe/H] ～ -2, followed by a rise again to a near-solar ratioof [Na/Fe] ～ 0 at the very metal-poor regime [Fe/H] ～ -3 to -4. These resultsare discussed in comparison with the previous observational studies along with thetheoretical predictions from the available chemical evolution models.     

Observing Boron in Metal-Poor Stars

A new sample of 7 stars ranging in metallicity from [Fe/H] = −2.0 to [Fe/H] = −0.75 has been analyzed in the boron spectral region. The targets were selected according to the availability (in the literature) of their lithium and beryllium abundances, because the simultaneous knowledge of LiBeB in the same targets is a powerful diagnostic for testing depletion and internal mixing predicted by different stellar structure models. Two stars (HD 94028 and HD 194598), characterized by similar Li contents, are found to have also similar B abundances, despite a 0.3 dex difference in their Be abundances claimed by Thorburn and Hobbs (1996). Four stars out of 7 are characterized by strongly depleted Li and Be abundances: 2 of them (HD 2665 and HD 3795) are also significantly B-depleted, while two others (HD 106516 and HD 221377) have near normal B abundances despite being depleted by a factor ≥ 10 in both Li and Be abundances. These stars place strong constraints on the nature and depth of the mixing processes responsible for their light element abundances. The 7th star (HD 160617) shows the remarkable aspect of deficient B, probably deficient Be, and completely normal Li. No stellar destruction mechanism can explain this. Rather, chemical inhomogeneities in the halo could be the cause. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Formation of Carbon-Enhanced Metal-Poor RR Lyrae Stars

Carbon-enhanced metal-poor (CEMP) stars are considered to be related to the first generation of stars, and responsible for the chemical evolution of the early Galaxy. More than half of them are in binaries, and could be explained by the binary evolution, but the formation channel of them is still not fully understood. Among the hundreds of CEMP stars, there are nine CEMP RR Lyrae stars identified, and at least seven of which are very likely not binaries. The usual binary star evolution channel is difficult to produce such a single star, particularly that of carbon enrichment. One way in which such a single star might be produced is the merger of a helium white dwarf with a Hertzsprung gap (HG) star. We use a stellar evolution program to calculate the models of the merger remnants, and find that the models can reproduce the observed distribution of these CEMP single RR Lyrae stars in terms of surface temperature, gravity, and carbon abundance. Hence, it is extremely possible that the helium white dwarf and HG star merger model is one of the formation channels of the metal-poor carbon-rich RR Lyrae stars.