Hierarchical Triples as Early Sources of $r$-process Elements

TL;DR

This paper proposes that hierarchical triple systems can lead to rapid neutron star mergers, explaining early r-process element enrichment in the universe, supported by hydrodynamic simulations showing increased ejecta and bright kilonovae.

Contribution

It introduces a new channel involving hierarchical triples that causes fast neutron star mergers, addressing the timing discrepancy in r-process enrichment.

Findings

Hierarchical triples can cause neutron star mergers within 10 Myr.

Eccentric mergers eject more mass, producing brighter kilonovae.

Scenario aligns with observed merger rates and early r-process enrichment.

Abstract

Neutron star mergers have been proposed as the main source of heavy $r$-process nucleosynthesis in the Universe. However, the mergers' significant expected delay after binary formation is in tension with observed very early $r$-process enrichment, e.g., in the dwarf galaxy Reticulum II. The LIGO and Virgo gravitational-wave observatories discovered two binary mergers with lighter companion masses ($\sim 2.6$ M$_\odot$) similar to the total mass of many binary neutron star systems in the Galaxy. The progenitor of such mergers could be a neutron star binary orbiting a black hole. Here we show that a significant fraction of neutron star binaries in hierarchical triples merge rapidly ($\gtrsim3\%$ within $\lesssim10$ Myr after neutron star formation) and could explain the observed very early $r$-process enrichment. The neutron star binary can become eccentric via von Zeipel-Kozai-Lidov oscillations, promoting a fast coalescence followed later by a merger of the low-mass black hole with the higher-mass black hole in the system. We show that this scenario is also consistent with an overall binary neutron star merger rate density of $\sim100$ Gpc$^{-3}$yr$^{-1}$ in such triples. Using hydrodynamic simulations we show that highly eccentric neutron star mergers dynamically eject several times more mass than standard mergers, with exceptionally bright kilonovae with an "early blue bump" as unique observational signatures.