EVOLUTION AND NUCLEOSYNTHESIS IN LOW-MASS ASYMPTOTIC GIANT BRANCH STARS. II. NEUTRON CAPTURE AND THE s-PROCESS

We present a new analysis of neutron capture occurring in low-mass asymptotic giant branch (AGB) stars su†ering recurrent thermal pulses. We use dedicated evolutionary models for stars of initial mass in the range 1 to 3 and metallicity from solar to half solar. Mass loss is taken into account with the M _ Reimers parameterization. The third dredge-up mechanism is self-consistently found to occur after a limited number of pulses, mixing with the envelope freshly synthesized 12C and s-processed material from the He intershell. During thermal pulses, the temperature at the base of the convective region barely reaches being the temperature in units of 108 K), leading to a marginal activation of T 8 D 3( T 8 the 22Ne(a, n)25Mg neutron source. The alternative and much faster reaction 13C(a, n)16O must then play the major role. However, the 13C abundance left behind by the H shell is far too low to drive the synthesis of the s-elements. We assume instead that at any third dredge-up episode, hydrogen downNows from the envelope penetrate into a tiny region placed at the top of the 12C-rich intershell, of the order of a few 10~4 At H reignition, a 13C-rich (and 14N-rich) zone is formed. Neutrons by the major 13C M _ . source are then released in radiative conditions at during the interpulse period, giving rise to an T 8 D 0.9 efficient s-processing that depends on the 13C pro-le in the pocket. A second small neutron burst from the 22Ne source operates during convective pulses over previously s-processed material diluted with fresh Fe seeds and H-burning ashes. The main features of the -nal s-process abundance distribution in the material cumulatively mixed with the envelope through the various third dredge-up episodes are dis- cussed. Contrary to current expectations, the distribution cannot be approximated by a simple exponen- tial law of neutron irradiations. The s-process nucleosynthesis mostly occurs inside the 13C pocket; the form of the distribution is built through the interplay of the s-processing occurring in the intershell zones and the geometrical overlap of di†erent pulses. The 13C pocket is of primary origin, resulting from proton captures on newly synthesized 12C. Conse- quently, the s-process nucleosynthesis also depends on Fe seeds, a lower metallicity favoring the pro- duction of the heaviest elements. This allows a wide range of s-element abundance distributions to be produced in AGB stars of di†erent metallicities, in agreement with spectroscopic evidence and with the Galactic enrichment of the heavy s-elements at the time of formation of the solar system. AGB stars of metallicity are the best candidates for the buildup of the main component, i.e., for the s- Z ^ 1 Z _ distribution of the heavy elements from the Sr-Y-Zr peak up to the Pb peak, as deduced by meteoritic and solar spectroscopic analyses. A number of AGB stars may actually show in their envelopes an s- process abundance distribution almost identical to that of the main component. Eventually, the astro- physical origin of mainstream circumstellar SiC grains recovered from pristine meteorites, showing a nonsolar s-signatures in a number of trace heavy elements, is likely identi-ed in the circumstellar envelopes of AGB stars of about solar metallicity, locally polluting the interstellar medium from which the solar system condensed. Subject headings: nuclear reactions, nucleosynthesis, abundances E stars: AGB and post-AGB E stars: evolution E stars: low-mass, brown dwarfs