High Lithium Abundance Connection with the Chromospheric Helium in Red Giants: Spectroscopic and Asteroseismic analyses

TL;DR

This study links high lithium levels and strong chromospheric helium lines in red giant stars, suggesting a shared origin related to the He-flash event during stellar evolution, with implications for understanding stellar activity and chemical enrichment.

Contribution

It provides the first observational evidence connecting lithium enrichment and chromospheric helium activity in red clump giants, highlighting the role of the He-flash in these phenomena.

Findings

High Li and strong He I lines are exclusive to red clump giants.

A decline in He I strength correlates with decreasing Li abundance among RC giants.

Younger, Li-rich RC giants are He-strong, while older, Li-normal RC giants are He-weak.

Abstract

We present a study of correlations between high Li abundances and strong chromospheric He I 10830 \AA\ absorption line strengths in Kepler field giant stars. Our sample includes 84 giants with detectable solar-like oscillations in their lightcurves, and their Li abundances come from the literature or were measured here using LAMOST medium-resolution spectra. Evolutionary phases are determined through asteroseismic analysis, with mixed-mode period spacing (\Delta P) used to infer the time evolution of RC giants. Near-infrared observations of the He I \lambda 10830 line were obtained with the high-resolution Habitable-zone Planet Finder (HPF) spectrograph on the Hobby-Eberly Telescope (HET). We find high Li abundances and strong He I lines exclusively among red clump (RC) giants, with their absence in red giant branch stars suggesting a shared origin linked to the He-flash. Additionally, a steady decline in He I strength with decreasing Li abundance among RC giants indicates a correlation between these properties. Older, Li-normal RC giants are He-weak, while most younger super-Li-rich giants are He-strong, suggesting temporal evolution of both phenomena. We hypothesize that the core He-flash and subsequent sub-flashes may enhance Li abundances in RC giant photospheres and trigger heightened chromospheric activity, leading to stronger He I \lambda 10830 \AA\ lines in younger RCs. Over time, post-He-flash, chromospheric activity diminishes, resulting in weaker He I lines in older, Li-normal RCs.