Astrophysical turbulence is an intricate phenomenon specifically for the high Reynolds numbers encountered in stellar interiors that allow a wide spectra of eddies to coexist with large scale, coherent motions. The complex 3D character of the ensuing dynamical pattern goes well beyond the grasp of present day analytical modelling. In this paper, we bounded to a simple, ideal case to show that semiconvective shear zones as recently defined and suggested as a working hypothesis by Maeder (1996), are a natural prediction of standard perturbation theory. Of course, the extension of this result to real systems of more general geometry cannot be assured, also because of the possible influence of restoring forces other than buoyancy, inertial and radiative damping effects, as presently considered (e.g. magnetic fields). Indeed, diffusion is such a fragile transport mechanism that a wealth of secondary processes can easily modify its effects.
According to the linear analysis, overadiabatic stellar layers stable to the modified Ledoux criterion are semiconvective, whilst stellar regions stable to the Schwarzschild criterion can become convective for sufficiently large shears. Steep chemical gradients in rapidly rotating stellar interiors are no longer an insuperable obstacle for the products of stellar nucleosynthesis, and can diffuse to the surface sustained by a slightly increased diffusion coefficient and spatially reduced semiconvective and radiative stable layers. Differential rotation may thus appear as the only viable mechanism capable to contrast large µ-barriers, allowing for an appreciable enrichment in metals at the surface of fast rotating massive stars within their MS lifetime.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998