Impact of neutron captures on the r-process: I. Propagation of statistical model uncertainties

Abstract

Background: The r-process is responsible for producing about half of the elements heavier than iron in the Universe, but the underlying nuclear physics is still largely inaccessible to experiments and relies on theoretical calculations. Purpose: We aim to quantify the impact of statistical model uncertainties on the r-process with a large set of neutron-capture rates within the Hauser-Feshbach framework with varied statistical model parameters. Propagating neutron-capture rates to r-process calculations of various astrophysical sites allows us to link abundance features to nuclear uncertainties. Methods: We calculate neutron-capture reactivities using different nuclear level density (NLD) and gamma-ray strength function (GSF) prescriptions. We study their impact on r-process calculations for individual trajectories, mass-integrated ejecta from a neutron-star-merger, and a broad set of 10,000 parametric trajectories with a wide range of electron fractions and entropies. Results: The final abundances are more sensitive to NLDs than to GSFs. Selected NLDs produce pronounced abundance changes between the lanthanides and the third peak. Although the variation is reduced in the mass-integrated total ejecta, a subpeak at A=172-178 and systematic abundance changes at A=180-194 persist. The parametric trajectories show that the NLD sensitivity is greater in the cold r-process regime, where neutron captures continue after photodissociations have become ineffective. Additional conditions with large NLD sensitivities are found at intermediate electron fractions, where small changes in the matter flow can lead to significant differences in the final abundances. Conclusions: Systematic differences in neutron-capture rates between NLD prescriptions affect the final r-process abundances. Therefore, improved experimental constraints and theoretical descriptions of NLDs in these regions are needed.