Fission and the formation of the r-process rare-earth abundance peak in neutron star mergers

Abstract:  The recent observations of the GW170817 electromagnetic counterpart suggest lanthanides were produced in this neutron star merger event. However many questions regarding heavy element production in mergers remain: can such events account for all the r-process lanthanide material observed in the galaxy? are precious metals such as gold produced in sufficient amounts? are actinides produced? where within the merger environment does nucleosynthesis occur and under what specific conditions? Such questions can only be answered with careful studies of the nuclear physics uncertainties affecting r-process calculations. Here I will discuss recent extended calculations of beta-delayed fission and their implications for r-process nucleosynthesis. The influence of fission fragment distributions will also be addressed with a particular emphasis on the unknown origin of the r-process rare-earth peak at A~164. Since the rare-earth peak is formed as the r-process path begins to draw closer to stability, the rare-earth nuclei contributing to peak formation will soon be within reach of nuclear physics experiments performed at, for example, the CPT at CARIBU and the upcoming FRIB. Here I will present the latest results for the masses found to produce the rare-earth peak in a low entropy accretion disk wind scenario and compare directly with recent mass measurements from the CPT at CARIBU. Such collaborative efforts between theory and experiment could soon be in a position to make definitive statements regarding the mechanism of rare-earth peak formation and thus the astrophysical site of the r process.