DEEP DIFFUSIVE MIXING IN GLOBULAR-CLUSTER RED GIANTS

Additional mixing between the hydrogen burning shell and the base of t he convective envelope in stars on their first ascent of the red giant branch is traditionally considered as a possible cause for the star-t o-star abundance variations in globular clusters. In the present paper we model this deep mixing by diffusion. Nuclear kinetics equations co upled with diffusion terms are solved to follow the evolution of stell ar surface abundances for isotopes taking part in reactions of the pp- chains, CNO-, NeNa- and MgAl-cycles. Our task is to reproduce the ''gl obal anticorrelation'' of [O/Fe] versus [Na/Fe] found recently in glob ular cluster and halo giants by Kraft et al. (1993) and the correlatio n of [C/Fe] with M(V) observed in M 92 (Langer et al. 1986). It is sho wn that both correlations can be accounted for by deep diffusive mixin g with parameters for mixing depth and rate adjusted individually for each correlation. The deep mixing in red giants with solar metallicity is demonstrated to allow the interpretation of anomalous O-17/O-16 an d O-18/O-16 isotope ratios in S and C asymptotic giant branch stars. I n the conclusion we propose an observational test for the discussed de ep mixing.