R-Process Nucleosynthesis in Neutron Star Merger With New Fission Model

Abstract

The astrophysical site for the r-process has not yet been uniquely identified. Neutron star mergers (NSMs) have recently received special attention as production sites for the r-process. The ejected matter from the NSMs is extremely neutron-rich (Y e < 0.1) and the r-process path proceeds along the neutron drip line and enters the region of fissile nuclei. In this situation, theoretical models of nuclear masses and fission modes are quite important. In this study, we carry out r-process nucleosynthesis simulations in NSMs based upon recent numerical simulations. We here constructed a nuclear reaction network code and utilize a new model for nuclear masses and a fission fragment distributions. In our nucleosynthesis simulations the final r-process elemental abundances exhibit a very flat distribution for A = 90 − 180. This due to the fact that several fission cycles occur in the extremely neutron-rich conditions of NSMs. Combining these results with magneto-rotationally driven core-collapse supernovae (CCSNe) that predict successful r-process abundance peaks at A ∼ 130 and 195, we find that NSMs can resolve the underproduction problems of such CCSN models for the isotopes just below and above the abundance peaks. We discuss the relative contributions to the solar-system r-process yields from CCSNe and NSMs.