Energy generation in convective shells of low-mass, low-metallicity asymptotic giant branch stars

We report on the nonnegligible energy released by the ¹³C(α, n)¹⁶O reaction and the related neutron capture reactions in interior convective shells developed during the thermal pulsation phase of asymptotic giant branch (AGB) stars of low mass and low metallicity. Over 10⁴ L_⊙ are generated within a thermal pulse convective shell from the combination of the ¹³C(α, n)¹⁶O, neutron capture, and beta decay reactions. The inclusion of this energy source in the theoretical evolution of an AGB thermal pulse is shown to alter the evolution of the convective shell boundaries and, hence, how the ¹³C is ingested into the convective shell. By iterating between a nucleosynthesis code and a stellar model, we find a "converged" model in which the neutron density arising from the ¹³C source is substantially higher and the overall mean exposure of the irradiated material is lower than cases where no energy is taken into account. With these modifications in s-process parameters, the calculated abundance ratios for r-only to s-only nuclei at the same mass number remains surprisingly constant, but still does not eliminate the overproduction of ⁹⁶Zr, which disagrees with observation.