NUCLEOSYNTHESIS AND MIXING ON THE ASYMPTOTIC GIANT BRANCH .2. CARBON AND BARIUM STARS IN THE GALACTIC DISK

We study the role played by nucleosynthesis processes in thermal pulse s and by mixing episodes (the third dredge-up) in determining the abun dances of intrinsic and extrinsic asymptotic giant branch (AGB) stars. This is done by comparing results from AGB models with observations o f s-process and CNO nuclei in C stars (N-type) and in various classes of Ba stars (Ba dwarfs, CH subgiants, and Ba II giants) with metallici ties typical of the disk population. The complementary information com ing from abundances of Li and Mg isotopes is also discussed. According to a generally accepted scenario, the main neutron source at the orig in of s-process nucleosynthesis is assumed to be the reaction C-13(alp ha, n)O-16; a minor contribution derives also from the marginal activa tion of the reaction Ne-22(alpha, n)Mg-25 at the end of each flash. Ma king use of the latest neutron-capture cross sections and parameterizi ng the amount of C-13 burnt per pulse, we compute the nucleosynthesis occurring in the He shell and the dredge-up of material to the surface according to recent AGB models. Using envelope abundances after the f irst dredge-up derived from observations of first-ascent red giants an d adopting standard prescriptions for mass loss, we succeed in fitting the photospheric compositions of C and Ba stars within their uncertai nties. Our results confirm that C stars (N-type) are evolutionary desc endants of normal (Tc-rich) S stars and are characterized by the same spread in mean neutron exposures (0.2-0.4 mbarn(-1)). As for the binar y Ba stars, their abundances are compatible with the from an AGB prima ry component to a dwarf or giant secondary. We show that several const raints, including the Mg isotope ratios and the neutron density derive d from the Rb/Sr ratio, require that s-processing occur in low-mass AG B stars but exclude the possibility that barium stars derive from prim aries of intermediate mass (3 less than or equal to M/M. less than or equal to 8) efficiently burning the neutron source Ne-22. The s-proces s-enriched binary Ba giants show mean neutron exposures covering a wid er range, reaching higher values (up to 1.0 mbarn(-1)) than for normal (single) C stars. An inverse correlation of the mean neutron exposure with metallicity is also present. Hence, the higher efficiency in s-p rocessing shown by several Ba stars is interpreted as an indication th at the metallicity range they cover is larger than for intrinsic AGB s tars commonly observed. In fact, if the amount of primary C-13 burnt i s roughly constant for the studied stars, their effectiveness in produ cing neutron-rich nuclei must increase (nonlinearly) toward lower meta l contents. In this scenario the exponential distributions of neutron exposures provided by low-mass AGB stars can account well for the s-pr ocess abundances observed in Population I AGB stars, with no need to i nvoke strong single neutron exposures, as sometimes suggested in the p ast.