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

We derive a new expression for the coefficient D-h of diffusion by horizontal turbulence in rotating stars. This new estimate can be up to two orders of magnitude larger than that 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 Mcircle dot star are calculated with the new coefficient. The new and large D-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.