Enhanced symmetry energy may bear universality of r-process abundances
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY(2023)
摘要
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 beta-decay rates, and ultimately depends on the binding energy of neutron-rich nuclei. The well-known Bethe-Weizs acker 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 abo v e the ground state - at higher temperatures of T approximate to 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 approximate to 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.
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关键词
symmetry energy,universality,r-process
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