Simulating Neutron Star Mergers as r-process Sources in Ultra Faint Dwarf Galaxies

TL;DR

This study uses cosmological simulations to explore how neutron star mergers contribute to r-process element enrichment in ultra-faint dwarf galaxies, highlighting the importance of merger location and natal kicks.

Contribution

It demonstrates that the environment of neutron star mergers significantly influences r-process enrichment patterns in ultra-faint dwarf galaxies, with implications for understanding observed elemental abundances.

Findings

R-process enrichment is sensitive to the merger environment.

Central mergers produce higher r-process levels similar to Reticulum II.

Outer galaxy mergers result in more extended and lower enrichment distributions.

Abstract

To explain the high observed abundances of r-process elements in local ultra- faint dwarf (UFD) galaxies, we perform cosmological zoom simulations that include r-process production from neutron star mergers (NSMs). We model star-formation stochastically and simulate two different halos with total masses $\approx 10^8 M_{\odot}$ at z = 6. We find that the final distribution of [Eu/H] vs. [Fe/H] is relatively insensitive to the energy by which the r-process material is ejected into the interstellar medium, but strongly sensitive to the environment in which the NSM event occurs. In one halo the NSM event takes place at the center of the stellar distribution, leading to high-levels of r-process enrichment such as seen in a local UFD, Reticulum II (Ret II). In a second halo, the NSM event takes place outside of the densest part of the galaxy, leading to a more extended r-process distribution. The subsequent star formation occurs in an interstellar medium with shallow levels of r-process enrichment which results in stars with low levels of [Eu/H] compared to Ret II stars even when the maximum possible r-process mass is assumed to be ejected. This suggests that the natal kicks of neutron stars may also play an important role in determining the r-process abundances in UFD galaxies, a topic that warrants further theoretical investigation.