Enhanced extra mixing in low-mass stars approaching the RGB tip and the problem of Li-rich red-clump stars

TL;DR

This paper investigates the origin of lithium enrichment in red giants, proposing that enhanced magnetic and rotational mixing processes near the RGB tip produce Li, supported by simulations and asteroseismology data.

Contribution

It introduces a new hypothesis that magnetic and rotational mixing mechanisms enhance Li production in RGB stars approaching the tip, explaining Li-rich red-clump stars.

Findings

RGB stars experience enhanced mixing near the RGB tip.

AMRI or magnetically-enhanced thermohaline convection likely cause Li enrichment.

The proposed mechanism aligns with hydrodynamics simulations and asteroseismology observations.

Abstract

A few percent of red giants are enriched in Lithium with $A(\mathrm{Li}) > 1.5$. Their evolutionary status has remained uncertain because these Li-rich giants can be placed both on the red-giant branch (RGB) near the bump luminosity and in the red clump (RC) region. However, thanks to asteroseismology, it has been found that most of them are actually RC stars. Starting at the bump luminosity, RGB progenitors of the RC stars experience extra mixing in the radiative zone separating the H-burning shell from the convective envelope followed by a series of convective He-shell flashes at the RGB tip, known as the He-core flash. The He-core flash was proposed to cause fast extra mixing in the stars at the RGB tip that is needed for the Cameron-Fowler mechanism to produce Li. We propose that the RGB stars are getting enriched in Li by the RGB extra mixing that is getting enhanced and begins to produce Li, instead of destroying it, when the stars are approaching the RGB tip. After a discussion of several mechanisms of the RGB extra mixing, including the joint operation of rotation-driven meridional circulation and turbulent diffusion, the Azimuthal Magneto-Rotational Instability (AMRI), thermohaline convection, buoyancy of magnetic flux tubes, and internal gravity waves, and based on results of (magneto-) hydrodynamics simulations and asteroseismology observations, we are inclined to conclude that it is the mechanism of the AMRI or magnetically-enhanced thermohaline convection, that is most likely to support our hypothesis.