1996A&A...308..773D

Additional mixing between the hydrogen burning shell and the base of the convective envelope in stars on their first ascent of the red giant branch is traditionally considered as a possible cause for the star-to-star abundance variations in globular clusters. In the present paper we model this deep mixing by diffusion. Nuclear kinetics equations coupled with diffusion terms are solved to follow the evolution of stellar surface abundances for isotopes taking part in reactions of the pp-chains, CNO-, NeNa- and MgAl-cycles. Our task is to reproduce the "global anticorrelation" of [O/Fe] versus [Na/Fe] found recently in globular cluster and halo giants by Kraft et al. (1993) and the correlation of [C/Fe] with M_V observed in M92 (Langer et al. 1986). It is shown that both correlations can be accounted for by deep diffusive mixing 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 ¹⁷O/¹⁶O and ¹⁸O/¹⁶O isotope ratios in S and C asymptotic giant branch stars. In the conclusion we propose an observational test for the discussed deep mixing.