s‐Process in low‐metallicity stars – I. Theoretical predictions
s‐Process in low‐metallicity stars – I. Theoretical predictions
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DOI：
10.1111/j.1365-2966.2010.16369.x
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发表时间：
2010-01
影响因子：
4.8
通讯作者：
S. Bisterzo;R. Gallino;O. Straniero;S. Cristallo;F. Generale;U. Torino;1. ViaP.Giuria;Torino
中科院分区：
文献类型：
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作者：
S. Bisterzo;R. Gallino;O. Straniero;S. Cristallo;F. Generale;U. Torino;1. ViaP.Giuria;Torino
在银河晕中观察到了大量富含 s 过程元素 (CEMP-s) 的碳增强贫金属恒星样本。这些低质量恒星（M≤0.9M）位于主序或红巨星阶段，并且不经历第三次挖掘（TDU）事件。 s过程的增强最有可能是由于处于渐近巨星分支（AGB）相（现在是白矮星）时，来自质量更大的伴星在双星系统中的吸积所致。为了解释光谱观察结果，需要更新 AGB 模型来详细跟踪核合成过程。我们提出了基于 AGB 恒星模型的核合成计算，该模型是通过 FRANEC（弗拉斯卡蒂·拉夫森-牛顿进化码）获得的，用于低初始恒星质量和低金属丰度。对于给定的金属丰度，s-过程元素丰度的广泛分布是通过改变 13 C 的量及其在袋中的分布来获得的，其中 13 C(�, n) 16 O 反应是主要中子源，在脉冲间相期间在辐射条件下释放中子。我们还考虑了第二个中子源 22 Ne(�, n) 25 Mg，在对流热脉冲期间部分激活。我们讨论了从碳到铋的元素的表面丰度，对于初始质量 M = 1.3 – 2 M�、低金属丰度（[Fe/H] 从 1 降至 3.6）的 AGB 模型以及不同的 13 个碳袋效率。我们特别分析了三个 s 过程峰的相对行为：轻 s（幻中子数 N = 50 处的 ls）、重 s（N = 82 处的 hs）和铅（N = 126）。为了表征 s 过程分布，需要两个 s 过程指标 [hs/ls] 和 [Pb/hs]。在在线材料中，我们提供了一组带有表面预测的数据表。我们的最终目标是提供一整套低质量低金属丰度s过程增强恒星的理论模型。在即将发表的论文中，我们将通过与 CEMP-s 恒星的观测结果进行比较来测试我们的结果。
A large sample of carbon enhanced metal-poor stars enriched in s-process elements (CEMP-s) have been observed in the Galactic halo. These stars of low mass (M � 0.9 M� ) are located on the main-sequence or the red giant phase, and do not undergo third dredge-up (TDU) episodes. The s-process enhancement is most plausibly due to accretion in a binary system from a more massive companion when on the asymptotic giant branch (AGB) phase (now a white dwarf). In order to interpret the spectroscopic observations, updated AGB models are needed to follow in detail the sprocess nucleosynthesis. We present nucleosynthesis calculations based on AGB stellar models obtained with FRANEC (Frascati Raphson-Newton Evolutionary Code) for low initial stellar masses and low metallicities. For a given metallicity, a wide spread in the abundances of the s-process elements is obtained by varying the amount of 13 C and its profile in the pocket, where the 13 C(�, n) 16 O reaction is the major neutron source, releasing neutrons in radiative conditions during the interpulse phase. We account also for the second neutron source 22 Ne(�, n) 25 Mg, partially activated during convective thermal pulses. We discuss the surface abundance of elements from carbon to bismuth, for AGB models of initial masses M = 1.3 – 2 M� , low metallicities ([Fe/H] from 1 down to 3.6) and for different 13 C-pockets efficiencies. In particular we analyse the relative behaviour of the three s-process peaks: light-s (ls at magic neutron number N = 50), heavy-s (hs at N = 82) and lead (N = 126). Two s-process indicators, [hs/ls] and [Pb/hs], are needed in order to characterise the s-process distribution. In the online material, we provide a set of data tables with surface predictions. Our final goal is to provide a full set of theoretical models of low mass low metallicity s-process enhanced stars. In a forthcoming paper, we will test our results through a comparison with observations of CEMP-s stars.