Propagation of Hauser-Feshbach uncertainty estimates to r-process nucleosynthesis: Benchmark of statistical property models for neutron rich nuclei far from stability

Abstract

Background: The multimessenger observations of the neutron star merger event GW170817 have re-energized the longstanding debate over the astrophysical origins of the most massive elements via the r-process nucleosynthesis. A key aspect of r-process studies is the ability to compare astronomical observations to theoretical calculations of nucleosynthesis yields in a meaningful way. To perform realistic nucleosynthesis calculations, understanding the uncertainty inherent in microphysics details such as nuclear reaction rates is as essential as understanding uncertainties in the modeling of the astrophysical environment. Purpose: In this work, we present an investigation of the uncertainty of neutron capture rate calculations using the Hauser-Feshbach model when they are extrapolated away from stability. This work aims to provide a quantitative measure of the dependability of Hauser Feshbach calculations when the models of statistical nuclear properties (level density and gamma-ray strength) that are tuned near or on stability are extrapolated to nuclei in an r-process network. Methods: We have selected from literature a number of level density and gamma-ray strength models that are appropriate for describing neutron-capture reaction cross sections and used them to calculate the neutron-capture rate of each nucleus participating in the network. All valid model combinations for temperatures between 10 −4 GK and 10 GK were used. In each calculation, we extracted E1 gamma-ray strengths and level densities and observed how these statistical properties affect the theoretical reaction rates. The resulting neutron capture rates were sampled with the Monte Carlo technique and used in r-process nucleosynthesis network calculations to map the range of possible results for the r-process abundances. Results: The results show that neutron capture rates calculated with the extrapolated models of statistical nuclear properties can vary by a couple of orders of magnitude between different calculations. Phenomenological models provide smoother results than semi-microscopic ones. They cannot, however, reproduce drastic changes in the nuclear structure such as shell closures. While the semi-microscopic models examined in this work do predict nuclear structure effects away from stability, it is not clear that these results are quantitatively accurate. The overall effect of the extrapolation uncertainty to the r-process nucleosynthesis yields has shown to be large enough to impede comparisons between observation and calculations. Conclusions: Microphysics details such as neutron capture rates affect in a significant way the outcome of nucleosynthesis calculations. The inherent uncertainty in extrapolations of the current Hauser-Feshbach theory away from stability presents a challenge to meaningful comparisons of the results of nucleosynthesis calculations with observations. Based on the results of this study it is suggested that progress in the development of better microscopic models of gamma strengths and level densities is urgently n...

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