Thermohaline instability and rotation-induced mixing. II. Yields of 3He for low- and intermediate-mass stars

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.