Lithium Evolution of Giant Stars Observed by LAMOST and Kepler

TL;DR

This study analyzes lithium abundance evolution in 1,848 giant stars using LAMOST and Kepler data, revealing insights into stellar interior processes and lithium production mechanisms during different evolutionary phases.

Contribution

It provides a comprehensive analysis of lithium evolution in giant stars, highlighting the role of helium flash and other mechanisms in lithium enrichment, based on a large observational sample.

Findings

Li depletion occurs along the RGB phase.

Li abundance increases after the RGB bump, indicating helium flash contributes to Li production.

Li-rich stars are found throughout the core He-burning phase, suggesting multiple formation mechanisms.

Abstract

Mapping lithium evolution for evolved stars will provide restrictions and constraints on the fundamental stellar interior physical processes, which further shed light on our understanding of the theory of stellar structure and evolution. Based on a sample of 1,848 giants with known evolutionary phases and lithium abundances from the LAMOST-\kepler{} and LAMOST-\emph{K}2 fields, we construct mass-radius diagrams to characterize the evolutionary features of lithium. The stars at red giant branch (RGB) phase show natural depletion along with their stellar evolution, particularly, there is no obvious crowd stars with anomalously high Li abundances near the bump. Most of the low-mass stars reaching their zero-age sequence of core-helium-burning (ZAHeB) have Li abundances around $\sim1.0$\,dex, which show an increase of Li abundance by $\sim0.6$\,dex compared to the stars above the bump of RGB. This suggests the helium flash can be responsible for moderate Li production. While for super Li-rich stars, some special mechanisms should be considered during helium flash. Other scenarios, such as merger, could also be interpretations given the Li-rich stars can be found at anytime during the steady state phase of core He-burning. During the core He-burning (HeB) phase, there is no indication of obvious lithium depletion.