Enhanced symmetry energy bears universality of the r-process

Abstract

The abundance of about half of the stable nuclei heavier than iron via the rapid neutron capture process or $r$-process is intimately related to the competition between neutron capture and $\beta$-decay rates, which ultimately depends on the binding energy of neutron-rich nuclei. The well-known Bethe-Weizs\"acker semi-empirical mass formula\cite{weiz,bethe} describes the binding energy of ground states -- i.e. nuclei with temperatures of $T\approx0$ MeV -- with the symmetry energy parameter converging between $23-27$ MeV for heavy nuclei. Here we find an unexpected enhancement of the symmetry energy at higher temperatures, $T\approx0.7-1.0$ MeV, from the available data of giant dipole resonances built on excited states. Although these are likely the temperatures where seed elements are created -- during the cooling down of the ejecta following neutron-star mergers\cite{mergersnucleo} or collapsars\cite{collapsar} -- the fact that the symmetry energy remains constant between $T\approx0.7-1.0$ MeV, suggests a similar trend down to $T\approx0.5$ MeV, where neutron-capture may start occurring. Calculations using this relatively larger symmetry energy yield a reduction of the binding energy per nucleon for heavy neutron-rich nuclei and inhibits radiative neutron-capture rates. This results in a substantial close in of the neutron dripline -- where nuclei become unbound -- which elucidates the long sought universality of heavy-element abundances through the $r$-process; as inferred from the similar abundances found in extremely metal-poor stars and the Sun.