Wave heating during the helium flash and lithium-enhanced clump stars

TL;DR

This study investigates whether internal gravity waves during the helium flash can cause lithium enhancement in red clump stars, finding that while waves induce convection, it does not reach the surface to explain observed lithium enrichment.

Contribution

The paper models wave-induced heating during the helium flash and concludes it cannot account for lithium enhancement in red clump stars, challenging previous hypotheses.

Findings

Waves deposit about 10^6 L_sun near the hydrogen shell.

Convection zones form but do not reach the stellar surface.

Wave heating does not explain lithium enhancement.

Abstract

Red Clump stars have been found to be enhanced in lithium relative to stars at the tip of the Red Giant Branch (TRGB), which is unexpected in current stellar models. At the TRGB, stars undergo the helium flash, during which helium burning briefly generates roughly $10^9 \, L_\odot$ of power and drives vigorous convection within the star's core. The helium-burning shell excites large fluxes of internal gravity waves. Here we investigate whether or not these waves can deposit enough heat to destabilize the hydrogen-burning shell, generate a convection zone there, and thereby drive the Cameron-Fowler process to enhance surface $^{7}\mathrm{Li}$. We study this with detailed stellar evolution models, and find that while the waves deposit $\sim 10^6 \, L_\odot$ near the hydrogen-burning shell, and while this generally does produce a convection zone, the resulting convection does not reach high enough to merge with the envelope, and so cannot explain enhancements to surface $^{7}\mathrm{Li}$.