Rubidium, zirconium, and lithium production in intermediate-mass asymptotic giant branch stars
Rubidium, zirconium, and lithium production in intermediate-mass asymptotic giant branch stars
复制标题
中等质量渐近巨分支星中铷、锆和锂的生产
DOI：
10.1051/0004-6361/201117896
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发表时间：
2012
影响因子：
6.5
通讯作者：
D. Yong
中科院分区：
文献类型：
--
作者：
M. Raai;M. Lugaro;A. Karakas;D. A. García;D. Yong
上下文最近对银河系中质量（> 3 M）渐近巨星分支（AGB）的大样本的调查显示，它们表现出高达100 - 1000倍太阳的铷（Rb）过剩。相比之下，这些恒星中的锆（Zr）与太阳丰度相比并不丰富。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。目标。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星星模型，并比较他们目前的观测约束。方法.我们计算的Rb，Zr和Li的表面丰度与3和6.5 M之间的质量和0.02和0.004之间的金属丰度的恒星模型。结果我们发现，Rb丰度增加，增加恒星质量，从观测推断，但我们无法匹配的最高观测到的[Rb/Fe]丰度。与Rb产生有关的中子俘获截面的反应速率和负责这些恒星内部中子产生的22 Ne（β，n）25 Mg反应速率的变化，只产生了表面Rb含量的适度变化，Rb含量为1.003 dex。部分混合区（PMZ）作为额外的中子源激活13C（β，n）16O反应，可显著提高Rb丰度。然而，这导致锆丰度超过目前的观测限制的上限。如果在AGB演化的最后阶段，第三次疏浚（TDU）效率保持与早期阶段一样高，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。
Context. A recent survey of a large sample of Galactic intermediate-mass (>3 M⊙) asymptotic giant branch (AGB) stars shows that they exhibit large overabundances of rubidium (Rb) up to 100‐1000 times solar. In contrast, zirconium (Zr) is not enriched compared to the solar abundances in these stars. 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. Aims. 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. Methods. We calculate the Rb, Zr, and Li surface abundances for stellar models with masses between 3 and 6.5 M⊙ and metallicities between 0.02 and 0.004. Results. We find that the Rb abundance increases with increasing stell ar mass, as is inferred from observations but we are unable to match the highest observed [Rb/Fe] abundances. Variations of the reaction rates of the neutron-capture cross sections involved with Rb production and the rate of the 22 Ne(�, n) 25 Mg reaction, responsible for neutron production inside these stars, yields only modest variations in the surface Rb content of≈ 0.3 dex. Inclusion of a partial mixing zone (PMZ) to activate the 13 C(�, n) 16 O reaction as an additional neutron source yields significant enhancemen ts 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) effi ciency 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.