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–Weizsäcker 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.

Electric dipole polarizability of low-lying excited states in atomic nuclei

“…In conclusion, we have developed new equations for the dipole polarizability of excited states with arbitrary angular momenta on equal footing to ground states through the connection of E1 and E2 matrix elements, the hydrodynamic model and second-order perturbation theory. We apply this framework and perform full 1ÿω SM calculations of 2 SM sand κ SM values up to 36 Ar, with reasonable agreement with previous SM calculations done up to 20 Ne. Except the anomalous cases of 17,18 O, results follow the global trend predicted by Migdal [5].…”

“…The understanding of how α E1 evolves as a function of excitation energy is relevant for nuclear structure physics [18] and nuclear astrophysics [20]. Average properties can be extracted from GDRs built on excited states by fitting the GDR energy and width parameters to data [20], using the second-sound hydrodynamic model [21] and assuming the validity of the Brink-Axel hypothesis [22,23].…”

“…The understanding of how α E1 evolves as a function of excitation energy is relevant for nuclear structure physics [18] and nuclear astrophysics [20]. Average properties can be extracted from GDRs built on excited states by fitting the GDR energy and width parameters to data [20], using the second-sound hydrodynamic model [21] and assuming the validity of the Brink-Axel hypothesis [22,23]. The latter seems validated below critical temperatures of T T c = 0.7 + 37.5/A MeV and angular momenta J J c = 0.6A 5/6 [24], where excited GDRs present similar parameters to their ground-state counterparts [25,26].…”