Diffusion in differentially rotating stars

A linear stability analysis of rotating Boussinesq flows including shear, μ-gradients and radiation losses leads to modification of the classical Ledoux criterion for convective equilibrium. The role of shear in spreading the radial extent of the convective layers is briefly investigated, and we found that some effects may be expected in the latest nuclear evolutionary stages of massive stars, whose short timescales and rather steep μ-barriers concur to hinder angular momentum redistribution within their condensed cores. Also, semiconvective shear zones (Maeder 1996) are predicted in those radiative shells where the available shear energy is not sufficient to completely overturn the stable thermal gradient. We then provide a prescription to compute a modified diffusion coefficient for passive scalars in presence of shear and thermal conductivity.