"Nuclear thermometers" reveal the origin of the universal r-process nucleosynthesis

TL;DR

This paper uses giant dipole resonances as nuclear thermometers to explore the conditions of r-process nucleosynthesis in neutron-star mergers, revealing an enhanced symmetry energy at relevant temperatures that explains the universality of heavy element abundances.

Contribution

It provides experimental and theoretical evidence for an increased symmetry energy at r-process temperatures, linking nuclear properties to the universal pattern of heavy element formation.

Findings

Giant dipole resonances support the Brink-Axel hypothesis as probes of nuclear temperature.

Enhanced symmetry energy at T=0.51 MeV influences neutron drip line and element synthesis.

Shell-model calculations confirm the temperature-dependent symmetry energy effects.

Abstract

The resembling behaviour of giant dipole resonances built on ground and excited states supports the validity of the Brink-Axel hypothesis and assigns giant dipole resonances as spectroscopic probes -- or ``nuclear thermometers'' -- to explore the cooling of the kilonova ejecta in neutron-star mergers down to the production of heavy elements beyond iron through the rapid-neutron capture or r-process. In previous work, we found a slight energy increase in the giant dipole resonance built on excited states at the typical temperatures of $1.0\gtrapprox T\gtrapprox0.7$ MeV where seed nuclei are produced, before ongoing neutron capture. Crucial data are presented here supporting an enhanced symmetry energy at $T=0.51$ MeV (or $5.9\times 10^9$ K) -- where the r-process occurs -- that lowers the binding energy in the Bethe-Weizs\"acker semi-empirical mass formula and results in the close in of the neutron drip line. Ergo, providing an origin to the universality of elemental abundances by limiting the reaction network for r-process nucleosynthesis. An enhanced symmetry energy away from the ground state is further supported by shell-model calculations of the nuclear electric dipole ({\sc E1}) polarizability -- inversely proportional to the symmetry energy -- as a result of the destructive contribution of the products of off-diagonal {\sc E1} matrix elements.