Enhanced symmetry energy may bear universality of r-process abundances

Abstract

The abundances of about half of the elements heavier than iron are subtly attuned by the rapid neutron capture process or r-process, which is intimately related to the competition between neutron capture, photo-disintegration, and β-decay rates, and ultimately depends on the binding energy of neutron-rich nuclei. The well-known Bethe–Weizsacker ¨ semi-empirical mass formula describes the binding energy of ground states – i.e. nuclei with temperatures of T = 0 MeV – with the symmetry energy parameter converging between 23 and 27 MeV for heavy nuclei. We find an unexpected enhancement of the symmetry energy well above the ground state – at higher temperatures of 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 nuclei are created – during the cooling down of the ejecta following neutron-star mergers or collapsars – the fact that the symmetry energy remains constant between T ≈ 0.7 and 1.0 MeV, may suggest an enhanced symmetry energy at lower temperatures, 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 drip line which may elucidate 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. Sensitivity studies of r-process network calculations have been performed using more sophisticated mass models.