An Analytical Model of Neutron Star Merger R-Process Enrichment in Reticulum II

Abstract

The source of rapid neutron capture process (r-process) enrichment in galaxies is a highly debated topic. The r-process is responsible for creating heavy metals such as gold, europium, and uranium in extremely neutron dense environments. While neutron star mergers (NSM) remain the only observed astronomical site of the r-process, the timescale, frequency, and retention of r-process created by NSMs are uncertain. In this work, we model an r-process enriched ultra faint dwarf galaxy known as Reticulum II. Reticulum II formed in the very early universe and its stars are fossils of early chemical evolution. Its unique r-process enrichment (72% of the stars) compared to other ultra faint dwarf galaxies can tell us whether NSMs are the sole source of r-process enrichment in the universe. We create an analytical model with star formation histories based on Simon et al. 2023 and NSM delay time distributions from current literature. Assuming fiducial values for the minimum delay time (α = −1.83 and minimum delay time of 184 Myr), occurrence rate of neutron star binaries (1 NSM per 70,000 M⊙), and current stellar mass (≈ 3300 M⊙), we find that the probability of NSMs being the source of r-process enrichment of Reticulum II is statistically improbable. For our fiducial values, we get a 0.7% likelihood that NSMs are responsible for the enrichment of Ret II’s stars. We conclude that an alternative source of the r-process is needed, such as collapsars.