General properties of astrophysical reaction rates in explosive nucleosynthesis

TL;DR

This paper examines the fundamental differences and stellar modifications of reaction rates for intermediate and heavy nuclei in explosive nucleosynthesis, highlighting the importance of excited states, Coulomb effects, and experimental data integration.

Contribution

It introduces a linear weighting approach for excited state contributions, identifies Coulomb suppression effects, and provides guidance on improving reaction rate predictions and uncertainties.

Findings

Excited state contributions can be linearly weighted despite Boltzmann populations.

Coulomb suppression can counteract the Q-value rule in certain reactions.

Uncertainties in neutron capture rates are larger than experimental errors suggest.

Abstract

Fundamental differences in the prediction of reaction rates with intermediate and heavy target nuclei compared to the ones with light nuclei are discussed, with special emphasis on stellar modifications of the rates. Ground and excited state contributions to the stellar rates are quantified, deriving a linear weighting of excited state contributions despite of a Boltzmann population of the nuclear states. A Coulomb suppression effect of the excited state contributions is identified, acting against the usual Q-value rule in some reactions. The proper inclusion of experimental data in revised stellar rates is shown, containing revised uncertainties. An application to the s-process shows that the actual uncertainties in the neutron capture rates are larger than would be expected from the experimental errors alone. Sensitivities of reaction rates and cross sections are defined and their application in reaction studies is discussed. The conclusion provides a guide to experiment as well as theory on how to best improve the rates used in astrophysical simulations and how to assess their uncertainties.