STELLAR MODELS WITH MICROSCOPIC DIFFUSION AND ROTATIONAL MIXING .1. APPLICATION TO THE SUN

The Yale stellar evolution code has been modified to include the combi ned effects of diffusion and rotational mixing on H-1, He-4, and the t race elements(3) He, Li-6, Li-7, and Be-9. The interaction between rot ational mixing and diffusion is studied by calculating a number of cal ibrated solar models. The rotational mixing inhibits the diffusion in the outer parts of the models, leading to a decrease in the envelope d iffusion by 25%-50%. Conversely, diffusion leads to gradients in mean molecular weight which can inhibit the rotational mixing. The degree t o which gradients in mean molecular weight inhibit the rotational mixi ng is somewhat uncertain. A comparison with the observed solar oblaten ess suggests that gradients in the mean molecular weight play a smalle r role in inhibiting the rotational mixing than previously believed. T his is reinforced by the fact that the model with the standard value f or the inhibiting effect of mean molecular weight on the rotational mi xing depletes no Li on the main sequence. This is in clear contrast to the observations. A reduction in the inhibiting effect of mean molecu lar weight gradients by a factor of 10 leads to noticeable main-sequen ce Li depletion.