Neutron Nucleosynthesis in a Low-Mass, Low-Metallicity AGB Star

Extract

Stellar evolution calculations confirm that semiconvection will occur below the convective envelope of a low-mass, low-metallicity AGB star, after a thermal pulse. These calculations show how semiconvection leads to the creation of a “¹³C layer” in the star, which can provide a potent source of neutrons (via the ¹³C[a, n]¹⁶0 reaction) in a convective shell during later evolution. The rate at which neutrons are released is largely determined by the rate at which the ¹³C layer is introduced into the convective shell. The ¹³C neutron source maintains neutron densities of 10⁹-10¹⁰ n/cm³ for ∼ 10 years. This provides a neutron exposure r=0.15 mb“¹ during most of the pulses calculated. Because of the strong filtering effect by light elements, only 10—20% of the neutrons produced will be captured by iron-seed nuclei, each such nucleus capturing 4–5 neutrons per pulse.