Stellar rotation: Evidence for a large horizontal turbulence and its effects on evolution

We derive a new expression for the coefficient $D_{\mathrm{h}}$ of diffusion by horizontal turbulence in rotating stars. This new estimate can be up to two orders of magnitude larger than given by a previous expression. As a consequence the differential rotation on an equipotential is found to be very small, which reinforces Zahn's hypothesis of shellular rotation. The role of the so--called $\mu$--currents, as well as the driving of circulation, are reduced by the large horizontal turbulence. Stellar evolutionary models for a 20 M${\odot}$ star are calculated with the new coefficient. The new and large $D_{\mathrm{h}}$ tends to limit the size of the convective core and at the same time it largely favours the diffusion of helium and nitrogen to the surface of rotating OB stars, a feature supported by recent observations.