The link between rare earth peak formation and the astrophysical site of the $r$ process

TL;DR

This paper introduces a Monte Carlo method to identify nuclear mass surface features responsible for the rare earth peak in the $r$ process, linking nuclear physics to astrophysical site conditions like supernovae and neutron star mergers.

Contribution

The study presents a novel Monte Carlo approach that self-consistently adjusts nuclear masses and decay rates to determine conditions for rare earth peak formation in different astrophysical environments.

Findings

Identifies distinct mass surface features for supernova and neutron star merger scenarios.

Shows that future measurements can distinguish between different $r$ process sites.

Successfully locates regions of enhanced nuclear stability responsible for the peak.

Abstract

The primary astrophysical source of the rare earth elements is the rapid neutron capture process ($r$ process). The rare earth peak that is seen in the solar $r$-process residuals has been proposed to originate as a pile-up of nuclei during the end of the $r$ process. We introduce a new method utilizing Monte Carlo studies of nuclear masses in the rare earth region, that includes self-consistently adjusting $\beta$-decay rates and neutron capture rates, to find the mass surfaces necessary for the formation of the rare earth peak. We demonstrate our method with two types of astrophysical scenarios, one corresponding conditions typical of core-collapse supernova winds and one corresponding to conditions typical of the ejection of the material from the tidal tails of neutron star mergers. In each type of astrophysical conditions, this method successfully locates a region of enhanced stability in the mass surface that is responsible for the rare earth peak. For each scenario, we find that the change in the mass surface has qualitatively different features, thus future measurements can shed light on the type of environment in which the $r$ process occurred.