Universality of r-process elemental abundances: a well-regulated conspiracy of astrophysical and nuclear properties

Abstract

The isovector distribution of thousands of products of electric dipole (E1) matrix elements connecting the ground and first excited states through virtual excitations in the giant dipole resonance (GDR) region is presented for the first time in selected p and sd shell nuclei from 1ℏω shell-model calculations. A smaller nuclear dipole polarizability for the first excited level with respect to the ground state arises from the destructive contribution of off-diagonal E1 matrix elements. This results in a larger symmetry energy consistent with data from GDRs built on excited states of heavier nuclei at temperatures of 0.5 ⪅ T ⪅ 1 MeV and previous calculations of the temperature dependence of the symmetry energy in medium-mass nuclei within the quasiparticle random phase approximation and the shell-model Monte Carlo. The corresponding reduction of the binding energy in the Bethe-Weizsäcker semi-empirical mass formula yields a drop of radiative neutron capture rates and the shift of the neutron drip line towards the line of stability; in turn, providing a plausible explanation for the origin of the universality of elemental abundances by sharply constraining the reaction network flow for r-process nucleosynthesis.