THE ROLE OF FISSION IN NEUTRON STAR MERGERS AND ITS IMPACT ON THE<i>r</i>-PROCESS PEAKS

Abstract

Comparing observational abundance features with nucleosynthesis predictions\nof stellar evolution or explosion simulations can scrutinize two aspects: (a)\nthe conditions in the astrophysical production site and (b) the quality of the\nnuclear physics input utilized. We test the abundance features of r-process\nnucleosynthesis calculations for the dynamical ejecta of neutron star merger\nsimulations based on three different nuclear mass models: The Finite Range\nDroplet Model (FRDM), the (quenched version of the) Extended Thomas Fermi Model\nwith Strutinsky Integral (ETFSI-Q), and the Hartree-Fock-Bogoliubov (HFB) mass\nmodel. We make use of corresponding fission barrier heights and compare the\nimpact of four different fission fragment distribution models on the final\nr-process abundance distribution. In particular, we explore the abundance\ndistribution in the second r-process peak and the rare-earth sub-peak as a\nfunction of mass models and fission fragment distributions, as well as the\norigin of a shift in the third r-process peak position. The latter has been\nnoticed in a number of merger nucleosynthesis predictions. We show that the\nshift occurs during the r-process freeze-out when neutron captures and\n{\\beta}-decays compete and an (n,{\\gamma})-({\\gamma},n) equilibrium is not\nmaintained anymore. During this phase neutrons originate mainly from fission of\nmaterial above A = 240. We also investigate the role of {\\beta}-decay\nhalf-lives from recent theoretical advances, which lead either to a smaller\namount of fissioning nuclei during freeze-out or a faster (and thus earlier)\nrelease of fission neutrons, which can (partially) prevent this shift and has\nan impact on the second and rare-earth peak as well.\n