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 3 He burning is efficient enough to produce RGB extra-mixing, two-dimensional 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 10 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 three-dimensional numerical simulations of thermohaline convection for the RGB case are clearly needed.