Fingering Convection in Red Giants Revisited

TL;DR

This study revisits fingering convection in Red Giants using a new 3D simulation-based prescription, finding it insufficient to explain observed surface abundances despite increased mixing coefficients.

Contribution

It introduces a new fingering convection model based on 3D simulations and evaluates its impact on Red Giant star models.

Findings

Fingering convection reaches the outer convective zone.

Mixing efficiency remains insufficient to match observations.

A two-order magnitude increase in the mixing coefficient is needed.

Abstract

Fingering (thermohaline) convection has been invoked for several years as a possible extra-mixing which could occur in Red Giant stars due to the modification of the chemical composition induced by nuclear reactions in the hydrogen burning zone. Recent studies show however that this mixing is not sufficient to account for the needed surface abundances. A new prescription for fingering convection, based on 3D numerical simulations has recently been proposed (BGS). The resulting mixing coefficient is larger than the ones previously given in the literature. We compute models using this new coefficient and compare them to previous studies. We use the LPCODE stellar evolution code with the GNA generalized version of the mixing length theory to compute Red Giant models and we introduce fingering convection using the BGS prescription. The results show that, although the fingering zone now reaches the outer dynamical convective zone, the efficiency of the mixing is not enough to account for the observations. The fingering mixing coefficient should be increased by two orders of magnitude for the needed surface abundances to be reached. We confirm that fingering convection cannot be the mixing process needed to account for surface abundances in RGB stars.