Photospheric helium abundance in cool giants: a comprehensive study

Abstract

High-resolution optical spectra of 16 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 red giant branch (RGB)-bump (KIC 9821622) to AGB phase, unlike that expected from the RGB-tip to red clump phase. A plausible scenario for the enrichment of He 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.