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

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

Apr 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...497..388g&link_type=abstract

Astrophysical Journal v.497, p.388

Astronomy and Astrophysics

Astronomy

387

Nuclear Reactions, Nucleosynthesis, Abundances, Stars: Agb And Post-Agb, Stars: Evolution, Stars: Low-Mass, Brown Dwarfs

Scientific paper

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&sun; 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 12C and s-processed material from the He intershell. During thermal pulses, the temperature at the base of the convective region barely reaches T8 ~ 3 (T8 being the temperature in units of 108 K), leading to a marginal activation of the 22Ne( alpha , n)25Mg neutron source. The alternative and much faster reaction 13C( alpha , n)16O must then play the major role. However, the 13C abundance l( 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 12C-rich intershell, of the order of a few 10-4 M&sun;. At H reignition, a 13C-rich (and 14N-rich) zone is formed. Neutrons by the major 13C source are then released in radiative conditions at T8 ~ 0.9 during the interpulse period, giving rise to an efficient s-processing that depends on the 13C profile 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 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 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 different pulses. The 13C pocket is of primary origin, resulting from proton captures on newly synthesized 12C. 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 ~= \frac {1}{2} Z&sun; 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.

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