ABSTRACT

Barium (Ba) stars are chemically peculiar stars that display in their atmospheres signatures of the slow neutron-capture (s-process) mechanism that operates within asymptotic giant branch (AGB) stars, an important contributor to the cosmic abundance. The observed chemical peculiarity in these objects is not due to self-enrichment, but to mass transfer between the components of a binary system. The atmospheres of Ba stars are therefore excellent astrophysical laboratories, providing strong constraints for the nucleosynthesis of the s-process in AGB stars. In particular, rubidium (Rb) is a key element for the s-process diagnostic because it is sensitive to the neutron density and hence its abundance points to the main neutron source of the s-process in AGB stars. We present Rb abundances for a large sample of 180 Ba stars from high-resolution spectra (R = 48 000), and we compare the observed [Rb/Zr] ratios with theoretical predictions from s-process models in AGB stars. The target Ba stars in this study display [Rb/Zr] < 0, showing that Rb was not efficiently produced by the activation of the branching points at ⁸⁵Kr and ⁸⁶Rb. Model predictions from the Monash and FRUITY datasets of low-mass (≲4 M_⊙) AGB stars are able to cover the Rb abundances observed in the program Ba stars. These observations indicate that the ¹³C(α,n)¹⁶O reaction is the main neutron source of the s-process in the low-mass AGB companions of the observed Ba stars. We have not found in the present study candidate companions for former IR/OH massive AGB stars.

中文翻译：

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钡星中的

摘要

钡（Ba）恒星是化学特有的恒星，在其大气中表现出缓慢的中子捕获（s-过程）机制的特征，该机制在渐近大分支（AGB）恒星内运行，这是宇宙丰度的重要贡献者。在这些物体中观察到的化学特性不是由于自身富集，而是由于二元体系各组分之间的传质。因此，Ba恒星的大气是极好的天体物理学实验室，为AGB恒星s-过程的核合成提供了强大的约束条件。特别地，rub（Rb）是金属元素的关键元素。过程诊断，因为它对中子密度敏感，因此其丰度指向AGB星中s过程的主要中子源。我们从高分辨率光谱（R  = 48 000）中给出了180个Ba恒星的大量样品的Rb丰度，并将观察到的[Rb / Zr]比与AGB恒星s过程模型的理论预测进行了比较。本研究中的目标Ba星显示[Rb / Zr] <0，表明Rb不能通过激活⁸⁵ Kr和⁸⁶ Rb的分支点来有效地产生。从低质量的莫纳什和水果数据集模型的预测结果（≲4中号_⊙）AGB恒星能够覆盖程序Ba恒星中观测到的Rb丰度。这些观察结果表明，在观测到的Ba星的低质量AGB伴星中，¹³ C（α，n）¹⁶ O反应是s过程的主要中子源。在本研究中，我们尚未找到前IR / OH大AGB星的候选伴星。