Neutron Star Mergers Might not be the Only Source of r-Process Elements in the Milky Way

TL;DR

This paper investigates whether neutron star mergers are the sole source of r-process elements in the Milky Way, suggesting additional early-universe sources are likely necessary to explain observed abundance trends.

Contribution

It challenges the neutron star merger-only scenario for r-process element production, proposing the existence of an early, transient source contributing significantly to Eu in the galaxy.

Findings

Neutron star mergers alone cannot explain the Eu abundance trend in the Milky Way.

Alternative sources active in the early universe may account for about 50% of Eu production.

Steeper delay-time distributions or early burst scenarios are inconsistent with observed galaxy types.

Abstract

Probing the origin of r-process elements in the universe represents a multi-disciplinary challenge. We review the observational evidence that probe the properties of r-process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-process elements present in our Galaxy. We found discrepancies with the neutron star (NS-NS) merger scenario. Assuming they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H]\,$>-1$ in the disk of the Milky Way cannot be reproduced while accounting for the delay-time distribution (DTD) of coalescence times ($\propto~t^{-1}$) derived from short gamma-ray bursts and population synthesis models. Steeper DTD functions ($\propto~t^{-1.5}$) or power laws combined with a strong burst of mergers before the onset of Type~Ia supernovae can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short gamma-ray bursts and Type~Ia supernovae occurring in early-type galaxies, and reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an extra production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this extra site could be responsible for roughly 50% of the Eu production in the early universe, before the onset of Type~Ia supernovae. Rare classes of supernovae could be this additional r-process source, but hydrodynamic simulations still need to ensure the conditions for a robust r-process pattern.