# Using failed supernovae to constrain the Galactic r-process element production

## TL;DR

This paper explores how combining neutron star mergers with neutron star-black hole mergers can better explain the observed distribution of r-process elements in the Galaxy, especially at low metallicities.

## Contribution

It introduces a model including neutron star-black hole mergers as a source of r-process elements, improving the match with observed galactic chemical abundances.

## Key findings

- Combined mergers reproduce observed r-process abundance scatter.
- Neutron star-black hole mergers contribute significantly at low metallicities.
- Reduced Fe production shifts r-process onset to lower metallicities.

## Abstract

Rapid neutron capture process (r-process) elements have been detected in a large fraction of metal-poor halo stars, with abundances relative to iron (Fe) that vary by over two orders of magnitude. This scatter is reduced to less than a factor of 3 in younger Galactic disc stars. The large scatter of r-process elements in the early Galaxy suggests that the r-process is made by rare events, like compact binary mergers and rare sub-classes of supernovae. Although being rare, neutron star mergers alone have difficulties to explain the observed enhancement of r-process elements in the lowest metallicity stars compared to Fe. The supernovae producing the two neutron stars already provide a substantial Fe abundance where the r-process ejecta from the merger would be injected. In this work we investigate another complementary scenario, where the r-process occurs in neutron star-black hole mergers in addition to neutron star mergers. Neutron star-black hole mergers would eject similar amounts of r-process matter as neutron star mergers, but only the neutron star progenitor would have produced Fe. Furthermore, a reduced efficiency of Fe production from single stars significantly alters the age-metallicity relation, which shifts the onset of r-process production to lower metallicities. We use the high-resolution [(20 pc)3/cell] inhomogeneous chemical evolution tool `ICE' to study the outcomes of these effects. In our simulations, an adequate combination of neutron star mergers and neutron star-black hole mergers qualitatively reproduces the observed r-process abundances in the Galaxy.