中等质量渐近巨星分支星中铷、锆和锂的产生
Rubidium, zirconium, and lithium production in intermediate-mass asymptotic giant branch stars
摘要：
最近对银河系中等质量（>3 Msun）渐近巨星分支（AGB）恒星的大样本的调查表明，它们表现出高达100- 1000倍太阳的铷（Rb）过剩。这些观测结果限制了我们对中等质量AGB星内部发生的慢中子捕获过程（S过程）的理论概念。这些恒星的锂（Li）丰度也有报道。在中等质量的AGB星中，Li可以通过发生在对流包层底部的质子捕获产生。由于这个原因，Rb、Zr和Li的观测结果对发生在同一颗恒星上的不同过程设置了互补的约束。我们提出的Rb，Zr和Li的丰度的预测计算的第一次同时在中等质量的AGB星模型，并比较它们目前的观测约束。我们发现Rb丰度随着恒星质量的增加而增加，这是从观测中推断的，但我们无法与观测到的最高[Rb/Fe]丰度相匹配。包括部分混合区（PMZ）激活13 C（a，n）16 O反应作为一个额外的中子源产生显着的增强Rb丰度。然而，这导致锆丰度超过目前的观测限制的上限。如果第三次疏浚（TDU）的效率保持在AGB演化的最后阶段，在早期阶段一样高，我们可以匹配所观察到的Rb丰度范围的最低值。我们预测的锂丰度，这是观察到的大的变化。最后，预测的Rb生产量随着金属丰度的降低而增加，与麦哲伦云AGB星的观测结果定性一致。然而，Z=0.008和Z=0.004的中等质量AGB星的恒星模型没有产生足够的Rb来匹配观测到的丰度。
英文摘要：
A recent survey of a large sample of Galactic intermediate-mass (>3 Msun) asymptotic giant branch (AGB) stars shows that they exhibit large overabundances of rubidium (Rb) up to 100--1000 times solar. These observations set constraints on our theoretical notion of the slow neutron capture process (s process) that occurs inside intermediate-mass AGB stars. Lithium (Li) abundances are also reported for these stars. In intermediate-mass AGB stars, Li can be produced by proton captures occuring at the base of the convective envelope. For this reason the observations of Rb, Zr, and Li set complementary constraints on different processes occurring in the same stars. We present predictions for the abundances of Rb, Zr, and Li as computed for the first time simultaneously in intermediate-mass AGB star models and compare them to the current observational constraints. We find that the Rb abundance increases with increasing stellar mass, as is inferred from observations but we are unable to match the highest observed [Rb/Fe] abundances. Inclusion of a partial mixing zone (PMZ) to activate the 13C(a,n)16O reaction as an additional neutron source yields significant enhancements in the Rb abundance. However this leads to Zr abundances that exceed the upper limits of the current observational constraints. If the third dredge-up (TDU) efficiency remains as high during the final stages of AGB evolution as during the earlier stages, we can match the lowest values of the observed Rb abundance range. We predict large variations in the Li abundance, which are observed. Finally, the predicted Rb production increases with decreasing metallicity, in qualitative agreement with observations of Magellanic Cloud AGB stars. However stellar models of Z=0.008 and Z=0.004 intermediate-mass AGB stars do not produce enough Rb to match the observed abundances.
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