The formation of the r-process rare-earth peak in neutron star mergers

The electromagnetic counterpart of the GW170817 neutron star merger event suggested lanthanides were synthesized. However lanthanide production in heavy element nucleosynthesis is subject to large uncertainties. For instance the rare-earth abundance peak, a feature of enhanced lanthanide production at A~164 in the solar r-process residuals, is not robustly produced in r-process calculations. The proposed dynamical mechanism of peak formation requires the r-process path to encounter a nuclear physics feature which may be within reach of experiments performed at, for example, the upcoming FRIB. We employ Markov Chain Monte Carlo studies to "reverse engineer" the nuclear masses capable of producing a peak compatible with the observed solar r-process abundances. Here I will compare neutron-rich mass measurements from the CPT at CARIBU with the results for the masses found in both "cold" and "hot" accretion disk wind and dynamical ejecta conditions. Such direct comparisons between theory and experiment could soon be in a position to make definitive statements regarding the astrophysical site of rare-earth peak formation.