The diffusional evolution of chemical composition in a solar model

We consider the basic physical processes resulting in a differential, microscopic redistribution of stellar matter, generally known as diffusion. The main effect of diffusion in the solar interior is a segregation of light and heavy elements in the gravitational field. As a result, the abundance of helium and heavy elements in the solar envelope is reduced, while it becomes enriched by hydrogen. We present estimates of the degree of settling for a sequence of evolutionary models via numerical solution of the generalized diffusion equation. The effect of the ion charge (in the approximation of full ionization) on the settling rate is studied in detail. Abundance variations are given for the centers of the models, as well as for the convective envelope of the modern Sun. We analyze the effects of the thermal and concentrational diffusion on the evolution of the chemical-composition profile. Quantitatively, the effect of thermal diffusion is not very large, but it leads to the appearance of new features in the hydrogen-abundance profile, namely, a discontinuity at the base of the convective zone. The effect of concentration diffusion is relatively small, and is appreciable only at the model center at late stages of the evolution, and also close to the base of the convective envelope. All the mechanisms studied are necessary components of a modern model for the internal structure and evolution of the Sun.