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

Context. The He-3 content of Galactic HII regions is very close to that of the Sun and the solar system, and only slightly higher than the primordial He-3 abundance as predicted by the standard Big Bang nucleosynthesis. However, the classical theory of stellar evolution predicts a high production of He-3 by low-mass stars, implying a strong increase of He-3 with time in the Galaxy. This is the well-known He-3 problem. Aims. We study the effects of thermohaline and rotation-induced mixings on the production and destruction of He-3 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-circle dot 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 He-3 and on the net yields. Results. Over the whole mass and metallicity range investigated, rotation-induced mixing lowers the He-3 production, as well as the upper mass limit at which stars destroy He-3. For low-mass stars, thermohaline mixing occuring beyond the RGB bump is the dominant process in strongly reducing the net He-3 yield compared to standard computations. Yet these stars remain net 3He producers. Conclusions. Overall, the net He-3 yields are strongly reduced compared to the standard framework predictions.