The Branchings of the Main s-process: Their Sensitivity to alpha-induced Reactions on 13C and 22Ne and to the Uncertainties of the Nuclear Network

TL;DR

This study analyzes how uncertainties in alpha-induced reactions on 13C and 22Ne affect the s-process nucleosynthesis in AGB stars, highlighting the impact on isotopic abundances and the importance of nuclear data precision.

Contribution

It provides a detailed assessment of the sensitivity of the main s-process to key nuclear reaction rates and identifies the reactions most influential on isotopic abundance predictions.

Findings

Uncertainty in 13C(alpha, n)16O has marginal effects when fully consumed.

Reduction in 13C(alpha, n)16O rate can increase residual 13C and affect neutron production.

Variations in 22Ne(alpha, n)25Mg rate by a factor of two still reproduce solar isotopic abundances.

Abstract

This paper provides a detailed analysis of the main component of the slow neutron capture process (the s-process), which accounts for the solar abundances of half of the nuclei with 90 <~ A <~ 208. We examine the impact of the uncertainties of the two neutron sources operating in low-mass asymptotic giant branch (AGB) stars: the 13C(alpha, n)16O reaction, which releases neutrons radiatively during interpulse periods (kT ~ 8 keV), and the 22Ne(alpha, n)25Mg reaction, partially activated during the convective thermal pulses (TPs). We focus our attention on the branching points that mainly influence the abundance of s-only isotopes. In our AGB models, the 13C is fully consumed radiatively during interpulse. In this case, we find that the present uncertainty associated to the 13C(alpha, n)16O reaction has marginal effects on s-only nuclei. On the other hand, a reduction of this rate may increase the amount of residual (or unburned) 13C at the end of the interpulse: in this condition, the residual 13C is burned at higher temperature in the convective zone powered by the following TP. The neutron burst produced by the 22Ne(alpha, n)25Mg reaction has major effects on the branches along the s path. The contributions of s-only isotopes with 90 <~ A <= 204 are reproduced within solar and nuclear uncertainties, even if the 22Ne(alpha, n)25Mg rate is varied by a factor of two. Improved beta-decay and neutron capture rates of a few key radioactive nuclides would help to attain a comprehensive understanding of the solar main component.