Thermohaline mixing in evolved low-mass stars

Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: cantiello@astro.uni-bonn.de

Received: 23 February 2010
Accepted: 27 May 2010

Abstract

Context. Thermohaline mixing has recently been proposed to occur in low-mass red giants, with large consequence for the chemical yields of low-mass stars.

Aims. We investigate the role of thermohaline mixing during the evolution of stars between 1  and 3 , in comparison with other mixing processes acting in these stars.

Methods. We use a stellar evolution code which includes rotational mixing, internal magnetic fields and thermohaline mixing.

Results. We confirm that during the red giant stage, thermohaline mixing has the potential to decrease the abundance of ³He, which is produced earlier on the main sequence. In our models we find that this process is working on the RGB only in stars with initial mass M 1.5 . Moreover we report that thermohaline mixing is also present during core He-burning and beyond, and has the potential to change the surface abundances of AGB stars. While we find rotational and magnetic mixing to be negligible compared to the thermohaline mixing in the relevant layers, the interaction of thermohaline motions with the differential rotation may be essential to establish the timescale of thermohaline mixing in red giants.

Conclusions. To explain the surface abundances observed at the bump in the luminosity function, the speed of the mixing process needs to be more than two orders of magnitude higher than in our models. However it is not clear if thermohaline mixing is the only physical process responsible for these surface-abundance anomalies. Therefore it is not possible at this stage to calibrate the efficiency of thermohaline mixing against the observations.