Neon-22 as a neutron source, light elements as modulators, and s-process nucleosynthesis in a thermally pulsing star

If the reaction 22Ne(a, n)25Mg is the major source of neutrons in the convective shell of a thermally pulsing star, the fraction of emitted neutrons that can be captured by 56Fe and its neutron-capture progeny is controlled by 22Ne, 25Mg, and the neutron-capture progeny of 22Ne and 25Mg. Adopting currently quoted neutroncapture cross sections and choosing the initial ratio of 22Ne to 56Fe to be within a factor of 2 of the solar system value, it is shown that from three to six neutrons are captured by 56Fe and its progeny for every fresh 56Fe engulfed by the convective shell during a thermal pulse. The resultant distribution of heavy s-process elements resembles the solar system distribution. Subject headings: abundances, stellar - convection - interiors, stellar - nucleosynthesis - stellar evolution