Can neutron star mergers alone explain the r-process enrichment of the Milky Way?

TL;DR

This paper investigates whether neutron star mergers alone can account for the r-process element enrichment in the Milky Way, highlighting the importance of merger timing, ejecta mass, and binary evolution models.

Contribution

It demonstrates that neutron star mergers could explain early r-process enrichment only under specific conditions related to merger delay times and ejecta masses, and identifies limitations in current binary population models.

Findings

Neutron star mergers can contribute to early r-process enrichment if they have short delay times.

Black hole-neutron star mergers may significantly influence early chemical enrichment.

Current binary population synthesis models fail to reproduce observed elemental abundances.

Abstract

Comparing Galactic chemical evolution models to the observed elemental abundances in the Milky Way, we show that neutron star mergers can be a leading r-process site only if at low metallicities such mergers have very short delay times and significant ejecta masses that are facilitated by the masses of the compact objects. Namely, black hole-neutron star mergers, depending on the black-hole spins, can play an important role in the early chemical enrichment of the Milky Way. We also show that none of the binary population synthesis models used in this paper, i.e., COMPAS, StarTrack, Brussels, ComBinE, and BPASS, can currently reproduce the elemental abundance observations. The predictions are problematic not only for neutron star mergers, but also for Type Ia supernovae, which may point to shortcomings in binary evolution models.