Neutron Star Mergers and their Impact on Second Generation Star Formation in the Early Universe

TL;DR

This study uses high-resolution cosmological simulations to explore how neutron star mergers influence the chemical enrichment of second-generation stars in early galaxies, focusing on r-process element distribution.

Contribution

It introduces a new neutron star merger model with variable explosion energies and delay times, revealing their effects on metal enrichment in early universe star formation.

Findings

High explosion energy results in 72% of Pop II stars being r-process enhanced.

Lower explosion energy leads to 80% of stars being enriched, but only 14% highly enriched.

Short delay times (10 Myr) produce minimal highly enriched stars, while longer delays (100 Myr) increase this fraction.

Abstract

The exact evolution of elements in the universe, from primordial to heavier elements produced via the r-process, is still under scrutiny. The supernova deaths of the very first stars led to the enrichment of their local environments, and can leave behind neutron stars (NS) as remnants. These remnants can end up in binary systems with other NSs, and eventually merge, allowing for the r-process to occur. We study the scenario where a single NS merger (NSM) enriches a halo early in its evolution to understand the impact on the second generation of stars and their metal abundances. We perform a suite of high resolution cosmological zoom-in simulations using Enzo where we have implemented a new NSM model varying the explosion energy and the delay time. In general, a NSM leads to significant r-process enhancement in the second generation of stars in a galaxy with a stellar mass of $\sim 10^5 \mathrm{M}_{\odot}$ at redshift 10. A high explosion energy leads to a Pop II mass fraction of 72% being highly enhanced with r-process elements, while a lower explosion energy leads to 80% being enhanced, but only 14% being highly enhanced. When the NSM has a short delay time of 10 Myr, only 5% of the mass fraction of Pop II stars is highly enhanced, while 64% is highly enhanced for the longest delay time of 100 Myr. This work represents a stepping stone towards understanding how NSMs impact their environments and metal abundances of descendant generations of stars.