Thermohaline instability and rotation-induced mixing

II. Yields of ³He for low- and intermediate-mass stars

¹ Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland
e-mail: nadege.lagarde@unige.ch
² IRAP, UMR 5277 CNRS and Université de Toulouse, 14, Av. E. Belin, 31400 Toulouse, France

Received: 21 July 2011
Accepted: 13 September 2011

Abstract

Context. The ³He content of Galactic HII regions is very close to that of the Sun and the solar system, and only slightly higher than the primordial ³He abundance as predicted by the standard Big Bang nucleosynthesis. However, the classical theory of stellar evolution predicts a high production of ³He by low-mass stars, implying a strong increase of ³He with time in the Galaxy. This is the well-known “³He problem”.

Aims. We study the effects of thermohaline and rotation-induced mixings on the production and destruction of ³He over the lifetime of low- and intermediate-mass stars at various metallicities.

Methods. We compute stellar evolutionary models in the mass range 1 to 6 M_⊙ for four metallicities, taking into account thermohaline instability and rotation-induced mixing. For the thermohaline diffusivity we use the prescription based on the linear stability analysis, which reproduces red giant branch (RGB) abundance patterns at all metallicities. Rotation-induced mixing is treated taking into account meridional circulation and shear turbulence. We discuss the effects of these processes on internal and surface abundances of ³He and on the net yields.

Results. Over the whole mass and metallicity range investigated, rotation-induced mixing lowers the ³He production, as well as the upper mass limit at which stars destroy ³He. For low-mass stars, thermohaline mixing occuring beyond the RGB bump is the dominant process in strongly reducing the net ³He yield compared to standard computations. Yet these stars remain net ³He producers.

Conclusions. Overall, the net ³He yields are strongly reduced compared to the standard framework predictions.