CORRELATED NUCLEOSYNTHESIS OF FLUORINE AND S-PROCESS ELEMENTS IN ASYMPTOTIC GIANT BRANCH STARS

The production of fluorine and s-process elements in conditions simula ting the helium-burning shell of asymptotic giant branch (AGB) stars i s investigated from parametric one-zone nucleosynthesis calculations. Our calculations account for the correlation between the overabundance s of these elements, as observed at the surface of S and C stars (Jori ssen et al. 1992, A&A 261, 164). A primary supply of C-13 is, however, required to produce F-19 and s-process elements at the observed level s through the operation of the C-13(alpha,n)O-16 neutron source. Our c alculations allow to put strong constraints on this primary supply. Th ey require a C-13 mass fraction of at least 0.003, with little sensiti vity to the stellar conditions. This value is about 30 times larger th an the secondary C-13 supply available at the end of the CNO cycle in solar-metallicity stars. The minimum N-14 mass fractions required are found to vary between about 6 10(-4) and 0.01, depending on the temper ature at which the fluorine production occurs.