Photospheric Helium Abundance in Cool Giants -- A Comprehensive Study

TL;DR

This study investigates helium and lithium enrichment in cool giant stars through high-resolution spectroscopy, revealing that helium enhancement is associated with lithium-richness and suggesting internal synthesis and dredging as possible mechanisms.

Contribution

First spectroscopic measurement of photospheric helium abundance in normal and Li-rich field giants, linking He and Li enrichment processes.

Findings

He-enhanced stars are also Li-rich, indicating a correlation.

Li-enrichment occurs across the giant branch, from RGB-bump to AGB.

Multiple scenarios may explain He and Li enrichment in giants.

Abstract

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.