# On Neutron Star Mergers as the Source of r-process Enhanced Metal Poor Stars in the Milky Way

## TL;DR

This study uses cosmological simulations to investigate whether neutron star mergers alone can account for the observed r-process enhanced metal-poor stars in the Milky Way, highlighting the need for higher merger rates or yields.

## Contribution

It models Galactic r-process enrichment with detailed mixing processes, providing new insights into the sufficiency of neutron star mergers as the sole source.

## Key findings

- Simulations underpredict the observed ratios of r-process enhanced stars by about an order of magnitude.
- No r-process enhanced stars form if the minimum coalescence time exceeds 100 Myr.
- Boosting the neutron star merger rate or yield is necessary to match observations.

## Abstract

We model the history of Galactic r-process enrichment using high-redshift, high-resolution zoom cosmological simulations of a Milky Way (MW) type halo. We assume that all r-process sources are neutron star mergers (NSMs) with a power law delay time distribution. We model the time to mix pollutants at subgrid scales, which allows us to to better compute the properties of metal poor (MP) and carbon enhanced metal poor (CEMP) stars, along with statistics of their r-process enhanced subclasses. Our simulations underpredict the cumulative ratios of r-process enhanced MP and CEMP stars (MP-r, CEMP-r) over MP and CEMP stars by about one order of magnitude, even when the minimum coalescence time of the double neutron stars ($t_{\rm min}$) is set to 1 Myr. No r-process enhanced stars form if $t_{\rm min}=100$ Myr. Our results show that even when we adopt the r-process yield estimates observed in GW170817, NSMs by themselves can only explain the observed frequency of r-process enhanced stars if either the birth rate of double neutron stars per unit mass of stars is boosted to $\approx10^{-4} M_\odot^{-1}$ or the Europium yield of each NSM event is boosted to $\approx 10^{-4} M_{\odot}$.