Reconstructing Masses of Merging Neutron Stars from Stellar $R$-Process Abundance Signatures

TL;DR

This study uses stellar $r$-process element ratios to infer the masses of neutron star binaries that produced heavy elements in metal-poor stars, offering new insights into their origins and properties.

Contribution

It introduces an inverse method to reconstruct neutron star merger progenitor masses from stellar abundance ratios, providing an independent perspective on $r$-process nucleosynthesis sources.

Findings

Neutron star mergers can explain all $r$-process material in metal-poor stars.

The method reproduces the current distribution of double neutron star systems.

Highly $r$-process enhanced stars suggest progenitors of very asymmetric neutron star mergers.

Abstract

Neutron star mergers (NSMs) are promising astrophysical sites for the rapid neutron-capture ("$r$-") process, but can their integrated yields explain the majority of heavy-element material in the Galaxy? One method to address this question has utilized a forward approach that propagates NSM rates and yields along with stellar formation rates, in the end comparing those results with observed chemical abundances of $r$-process-rich, metal-poor stars. In this work, we take the inverse approach by utilizing $r$-process-element abundance ratios of metal-poor stars as input to reconstruct the properties---especially the masses---of the neutron star (NS) binary progenitors of the $r$-process stars. This novel analysis provides an independent avenue for studying the population of the original neutron star binary systems that merged and produced the $r$-process material incorporated in Galactic metal-poor halo stars. We use ratios of elements typically associated with the limited-$r$ process and the actinide region to those in the lanthanide region (i.e., Zr/Dy and Th/Dy) to probe the NS masses of the progenitor merger. We find that NSMs can account for all $r$-process material in metal-poor stars that display $r$-process signatures, while simultaneously reproducing the present-day distribution of double-NS (DNS) systems. However, the most $r$-process enhanced stars (the $r$-II stars) on their own would require progenitor NSMs of very asymmetric systems that are distinctly different from present ones in the Galaxy. As this analysis is model-dependent, we also explore variations in line with future expectation regarding potential theoretical and observational updates, and comment on how these variations impact our results.