Neutron Capture Elements in s-Process-Rich, Very Metal-Poor Stars
Neutron Capture Elements in s-Process-Rich, Very Metal-Poor Stars
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DOI：
10.1086/323230
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发表时间：
2001-07
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影响因子：
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通讯作者：
W. Aoki;S. Ryan;J. Norris;T. Beers;H. Ando;N. Iwamoto;T. Kajino;G. Mathews;M. Fujimoto
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文献类型：
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作者：
W. Aoki;S. Ryan;J. Norris;T. Beers;H. Ando;N. Iwamoto;T. Kajino;G. Mathews;M. Fujimoto
我们报道了两颗富碳、贫金属的恒星LP625-44和LP706-7的中子俘获元素(包括铅)的丰度估计，以及它们的核合成模型。这些恒星是亚巨星，其表面丰度很可能受到伴星渐近巨星(AGB)质量转移的强烈影响，这些恒星后来演化为白矮星。对形成S过程最终丰度峰的铅的探测，使从锶(Z=38)到铅(Z=82)的丰度模式与AGB模型的预测相比较。所得到的化学成分对AGB恒星模型以及低金属丰度的S过程核合成提供了很强的约束。本文详细介绍了LP 625-44中16个中子俘获元素的丰度分析，包括某些元素的超精细分裂和谱线的同位素位移的影响。通过对以前观测到的光谱的重新分析，也得到了LP706-7的铅丰度。我们使用S过程的参数模型研究了产生这些天体丰度比的核合成途径的特征，而不采用任何特定的恒星模型。中子暴露量τ估计约为0.7mbarn-1，明显大于最适合太阳系物质的值，但与中等金属贫化的AGB星模型的预测值一致。这一数值除受锶和钡丰度的限制外，还受到铅丰度的严格限制。我们还发现，观测到的丰度模式可以用几次重复的中子照射来解释，并且在随后的两次照射中处理的物质的重叠很小(重叠因子r~0.1)。
We report abundance estimates for neutron capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly affected by mass transfer from companion asymptotic giant branch (AGB) stars that have since evolved to white dwarfs. The detections of Pb, which forms the final abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z = 38) to Pb (Z = 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron capture elements in LP 625-44, including the effects of hyperfine splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a reanalysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any specific stellar model. The neutron exposure τ is estimated to be about 0.7 mbarn-1, significantly larger than that which best fits solar system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also find that the observed abundance pattern can be explained by a few recurrent neutron exposures and that the overlap of the material that is processed in two subsequent exposures is small (the overlap factor r ~ 0.1).