Joint Discussion 5 - Mixing and Diffusion in Stars: Introduction and Brief Overview

Abstract

Our understanding of transport processes in stars is still in its infancy. First consider overshooting. There is now ample evidence from, e.g., the best-observed binary stars and fits of models to observed colour-magnitude (C-M) diagrams that some amount of mixing must occur beyond the classical boundary of the convective core set by the Schwarzschild criterion. Such mixing can be achieved by convective penetration; however, despite impressive progress on the theoretical front, a rigorous prediction of the extent of overshooting has not yet been achieved. As a result, most stellar evolutionary calculations have simply opted to extend convective core sizes by an amount (usually measured as a fraction of a pressure scale height) that produces reasonable consistency with observations. But how realistic are such models? To what extent does the overshooting depend on mass and/or evolutionary state? What is the role of rotation in determining the size of convective cores? How much does rotation-induced meridional circulation contribute to such mixing?