UNIFIED STELLAR MODELS AND CONVECTION IN COOL STARS

The formulation of boundary conditions can have a significant influenc e on the solution of a system of differential equations. It is therefo re important to apply a most realistic representation of the surface b oundary conditions to the equations of stellar structure and evolution . With respect to previous models that usually employ some estimate of the surface temperature drawn from the Eddington approximation, a sig nificant improvement of the outer boundary conditions is achieved by c onnecting models of stellar atmospheres to stellar structure models. U p to now stellar evolution calculations for late-type stars are calibr ated using the well-observed properties of the present Sun. Including the physics of a plane-parallel atmospheric stratification it is neces sary to account for a consistent description of the convective energy transfer in the outer layers of a cool star. At this step an apparent contradiction of the observations must be resolved: spectroscopic anal ysis of the Balmer lines emerging from solar-type stars using line-bla nketed model atmospheres are usually carried out with reference to the Bohm-Vitense convection theory. To fit simultaneously the profiles of H alpha and H beta as well as higher series members a small mixing-le ngth parameter alpha = l/H-p = 0.5 is required. Models calibrated to t he present Sun instead imply that the internal structure of the Sun fo llows a substantially higher value of alpha = 1.5. This discrepancy ca nnot be removed in the context of Bohm-Vitense's convection theory. It is shown that the convection model of Canuto & Mazzitelli fits both t he observed present Sun and the Balmer lines with a single common mixi ng-length parameter. The convection theory of Canuto & Mazzitelli thus offers for the first time a unified physical model of the Sun that is valid from the center to the upper photosphere.