Fingering convection in red giants revisited

¹ Instituto de Astrofísica de La Plata (UNLP – CONICET), Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, 1900 La Plata, Argentina
e-mail: fcw@fcaglp.unlp.edu.ar
² Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
³ CNRS, IRAP, 14 avenue Edouard Belin, 31400 Toulouse, France

Received: 10 July 2014
Accepted: 15 September 2014

Abstract

Context. Fingering (thermohaline) convection has been invoked for several years as a possible extra-mixing which could occur in red giant stars; it is 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.

Aims. A new prescription for fingering convection, based on 3D numerical simulations has recently been proposed. The resulting mixing coefficient is larger than those previously given in the literature. We compute models using this new coefficient and compare them to previous studies.

Methods. We used the LPCODE stellar evolution code with a generalized version of the mixing length theory to compute red giant models and we introduce fingering convection using the BGS prescription.

Results. 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.

Conclusions. We confirm that fingering convection cannot be the mixing process needed to account for surface abundances in red giant branch stars.