通往锂的条条大路：富锂红巨星的形成路径
Many Roads Lead to Lithium: Formation Pathways For Lithium-Rich Red Giants
AI总结：
本研究分析GALAH DR3中1155颗富锂巨星，通过对比锂正常伴星样本，发现快速自转、s过程丰度异常及红团簇分布等特征，表明双星自转加速和质量转移是锂富集的主要机制。
摘要：
恒星模型预测，恒星内部的锂（Li）在第一次挖掘阶段会被摧毁，然而仍有1.2%的红巨星星是富锂的。我们旨在通过分析GALAH DR3中的1155颗富锂巨星（A(Li) $\geq$ 1.5）来揭示这一星族的可能起源。为了揭示富锂恒星的奇特特征，我们构建了一个具有匹配演化状态和基准超新星丰度的锂正常（doppelgänger）恒星参考样本。比较富锂星和doppelgänger星的光谱，揭示了与速度展宽测量相关的H-$\alpha$和Ca三重线谱线轮廓的系统性差异。我们还发现，与doppelgänger星相比，富锂星中快速自转者（$v_\textrm{broad} \gtrsim 20$ km s$^{-1}$的占2%）的数量是其两倍。平均而言，对于一部分元素，富锂星的丰度高于其doppelgänger星，且位于红巨分支基部的富锂星相对于其doppelgänger星具有更高的平均$s-$过程丰度（Ba、Y、Zr的$\geq 0.05$ dex）。来自中等质量AGB伴星的外部质量转移可以解释这一特征。额外的伴星分析排除了在$\gtrsim 7$ AU处质量比$\gtrsim 0.5$的双星。我们还发现，高度富钡的恒星在富锂星族中缺失，这可能是由于低质量AGB伴星阻碍了锂的富集。最后，我们证实了红团簇上富锂星的普遍性随锂含量增加而增加，这支持了锂增强的演化状态机制。因此，包括双星自转加速和质量转移在内的多种因素可能是锂富集的机制。
英文摘要：
Stellar models predict that lithium (Li) inside a star is destroyed during the first dredge-up phase, yet 1.2% of red giant stars are Li-rich. We aim to uncover possible origins of this population, by analysing 1155 Li-rich giants (A(Li) $\geq$ 1.5) in GALAH DR3. To expose peculiar traits of Li-rich stars, we construct a reference sample of Li-normal (doppelgänger) stars with matched evolutionary state and fiducial supernova abundances. Comparing Li-rich and doppelgänger spectra reveals systematic differences in the H-$α$ and Ca-triplet line profiles associated with the velocity broadening measurement. We also find twice as many Li-rich stars appear to be fast rotators (2% with $v_\textrm{broad} \gtrsim 20$ km s$^{-1}$) compared to doppelgängers. On average, Li-rich stars have higher abundances than their doppelgängers, for a subset of elements, and Li-rich stars at the base of RGB have higher mean $s-$process abundances ($\geq 0.05$ dex for Ba, Y, Zr), relative to their doppelgängers. External mass-transfer from intermediate-mass AGB companions could explain this signature. Additional companion analysis excludes binaries with mass ratios $\gtrsim$ 0.5 at $\gtrsim$ 7 AU. We also discover that highly Ba-enriched stars are missing from the Li-rich population, possibly due to low-mass AGB companions which preclude Li-enrichment. Finally, we confirm a prevalence of Li-rich stars on the red clump that increases with lithium, which supports an evolutionary state mechanism for Li-enhancement. Multiple culprits, including binary spin-up and mass-transfer, are therefore likely mechanisms of Li-enrichment.
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