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 β -decay rates, which ultimately depends on the binding energy of neutron-rich nuclei. The well-known Bethe-Weizsäcker semi-empirical mass formula [1,2] describes the binding energy of ground states -i.e. nuclei with temperatures of T ≈ 0 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 ≈ 0.7 -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[3] or collapsars[4] -the fact that the symmetry energy remains constant between T ≈ 0.7 -1.0 MeV, suggests a similar trend down to T ≈ 0.5 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 heavyelement abundances through the r-process; as inferred from the similar abundances found in extremely metal-poor stars and the Sun.