Numerical Simulations of Thermohaline Convection: Implications for Extra-Mixing in Low-Mass RGB Stars

Low-mass stars are known to experience extra-mixing in their radiative zones on the red-giant branch (RGB) above the bump luminosity. To determine if the salt-fingering transport of chemical composition driven by 3He burning is efficient enough to produce RGB extra-mixing, 2D numerical simulations of thermohaline convection for physical conditions corresponding to the RGB case have been carried out. We have found that the effective ratio of a salt-finger's length to its diameter a_eff < 0.5 is more than ten times smaller than the value needed to reproduce observations (a_obs > 7). On the other hand, using the thermohaline diffusion coefficient from linear stability analysis together with a=a_obs is able to describe the RGB extra-mixing at all metallicities so well that it is tempting to believe that it may represent the true mechanism. In view of these results, follow-up 3D numerical simulations of thermohaline convection for the RGB case are clearly needed.