Thermohaline instability and rotation-induced mixing

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 6M\odot 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.