标题： 光球层氦丰度在冷巨星中的研究——一项综合研究 显示英文标题

标题： Photospheric Helium Abundance in Cool Giants -- A Comprehensive Study

摘要： 高分辨率光学光谱分析了十六个红巨星、两个早期渐近巨星分支（AGB）恒星和两个超巨星，这些恒星的锂（Li）丰度没有/极少到超丰富，以研究氦（He）增强。其中八个巨星的光谱来自喜马拉雅钱德拉望远镜，其余目标恒星的光谱数据则从多个公共档案中收集。我们对目标恒星的详细丰度分析涉及对大约20种元素的恒星参数和丰度的确定，其中包括关键丰度的He、Li、C、N、O以及12C/13C比值。通过Mg I线和MgH带得出的镁丰度差异，以及通过C I和CH带得出的碳丰度差异，作为轻微氢缺乏/氦增强的线索。通过这种分析，发现四个红巨星、一个早期AGB恒星和一个超巨星具有氦增强。所有这些氦增强的恒星都被发现是超锂丰富的，除了超巨星。由于富含氦的红巨星也是富锂的，这表明氦增强伴随着锂增强，但反之则不成立。这是首次对正常和富锂场巨星光球层氦丰度的光谱测量。锂增强现象在从RGB凸起（KIC 9821622）到AGB阶段的整个巨星分支上被观测到，这与从RGB尖端到RC相的预期不同。对于巨星中氦以及锂的富集，一种可能的场景是在巨星内部新鲜合成锂，并与氦一起从深层物质带到表面。然而，可能存在多种情景同时发生。对氦和锂富集以及其它关键元素的分析提供了更多的见解，以解开巨星中锂富集之谜。 显示英文摘要

摘要： High-resolution optical spectra of sixteen red giants, two early asymptotic giant branch (AGB) stars, and two supergiants, having no/minimal to super lithium (Li)-rich abundances, are analyzed to investigate the helium (He)-enhancement. The spectra of eight giants were obtained from the Himalayan Chandra Telescope, and for the rest of the program stars the spectral data were collected from various public archives. Our detailed abundance analyses of the program stars involve the determination of stellar parameters and abundances for about 20 elements among the key abundances of He, Li, C, N, O and the 12C/13C ratios. The difference in the Mg-abundance derived from Mg I lines and the MgH band, and the difference in carbon abundance from C I and the CH-band, are used as a clue to the mild hydrogen-deficiency/helium-enhancement. From this analysis, four red giants, an early-AGB star and a supergiant star were found to be enhanced in helium. All these He-enhanced stars are also found to be super Li-rich except for the supergiant. Since the He-rich red giants are Li-rich as well, this implies, that He-enrichment is accompanied by Li-enrichment but not vice-versa. This is the first spectroscopic measurement of photospheric He-abundance in normal and Li-rich field giants. The Li-enrichment is observed across the giant branch from RGB-bump (KIC 9821622) to AGB phase, unlike that expected from the RGB-tip to RC-phase. A plausible scenario for the enrichment of He and as well as Li in giants is the fresh synthesis of Li in the interiors of giants and dredging up along with He to the surface, from deeper layers. However, there could be multiple scenarios operating in tandem. This analysis of He- and Li-enrichment along with other key elements provides more insights to decipher the mystery of Li-enrichment in giants.