Evolution and Nucleosynthesis in Low‐Mass Asymptotic Giant Branch Stars. II. Neutron Capture and the s‐Process

Abstract

We present a new analysis of neutron capture occurring in low-mass asymptotic giant branch (AGB) stars suffering recurrent thermal pulses. We use dedicated evolutionary models for stars of initial mass in the range 1 to 3 M ☉ and metallicity from solar to half solar. Mass loss is taken into account with the 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 12 C and s -processed material from the He intershell. During thermal pulses, the temperature at the base of the convective region barely reaches T 8 ~ 3 ( T 8 being the temperature in units of 10 8 K), leading to a marginal activation of the 22 Ne(α, n ) 25 Mg neutron source. The alternative and much faster reaction 13 C(α, n ) 16 O must then play the major role. However, the 13 C 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 downflows from the envelope penetrate into a tiny region placed at the top of the 12 C-rich intershell, of the order of a few 10 -4 M ☉ . At H reignition, a 13 C-rich (and 14 N-rich) zone is formed. Neutrons by the major 13 C source are then released in radiative conditions at T 8 ~ 0.9 during the interpulse period, giving rise to an efficient s -processing that depends on the 13 C profile in the pocket. A second small neutron burst from the 22 Ne source operates during convective pulses over previously s -processed material diluted with fresh Fe seeds and H-burning ashes. The main features of the final s -process abundance distribution in the material cumulatively mixed with the envelope through the various third dredge-up episodes are discussed. Contrary to current expectations, the distribution cannot be approximated by a simple exponential law of neutron irradiations. The s -process nucleosynthesis mostly occurs inside the 13 C 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 different pulses. The 13 C pocket is of primary origin, resulting from proton captures on newly synthesized 12 C. Consequently, the s -process nucleosynthesis also depends on Fe seeds, a lower metallicity favoring the production of the heaviest elements. This allows a wide range of s -element abundance distributions to be produced in AGB stars of different 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 Z ≃ Z ☉ are the best candidates for the buildup of the main component, i.e., for the s -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 astrophysical origin of mainstream circumstellar SiC grains recovered from pristine meteorites, showing a nonsolar s -signatures in a number of trace heavy elements, is likely identified in the circumstellar envelopes of AGB stars of about solar metallicity, locally polluting the interstellar medium from which the solar system condensed.