A helium-flash-induced mixing event can explain the lithium abundances of red clump stars

TL;DR

This paper proposes that a helium-flash-induced mixing event explains the observed lithium abundance changes in low-mass red clump stars, linking stellar evolution processes with surface chemical composition.

Contribution

It introduces a stellar evolution model with a mixing prescription showing how the helium core flash enhances surface lithium in red clump stars.

Findings

Lithium abundance increases after the helium core flash.

Mixing of $^7$Be during the flash explains observed lithium levels.

Internal gravity waves may trigger the mixing process.

Abstract

Observations demonstrate that the surface abundance of $^7{\rm Li}$ in low-mass stars changes dramatically between the tip of the red giant branch and the red clump. This naturally suggests an association with the helium core flash, which occurs between these two stages. Using stellar evolution models and a simple, ad hoc mixing prescription, we demonstrate that the $^7{\rm Li}$ enhancement can be explained by a brief chemical mixing event that occurs at the time of the first, strongest He sub-flash. The amount of $^7{\rm Be}$ already present above the H-burning shell just before the flash, once it mixes into the cooler envelope and undergoes an electron capture converting it to $^7{\rm Li}$, is sufficient to explain the observed abundance at the red clump. We suggest that the excitation of internal gravity waves by the vigorous turbulent convection during the flash may provide a physical mechanism that can induce such mixing.