Compact Stellar Binary Assembly in the First Nuclear Star Clusters and r-Process Synthesis in the Early Universe

TL;DR

This paper proposes that early universe nuclear star clusters could produce enough compact binary mergers to explain the observed r-process element abundances in ancient, metal-poor stars, challenging previous neutron star merger models.

Contribution

It introduces a new scenario where dynamically assembled compact binaries in high-redshift nuclear star clusters are a significant source of r-process elements in early stars.

Findings

High formation rate of compact binaries in early star clusters

Sufficient r-process material ejected per merger event

Explains observed scatter of Eu in CEMP-r stars

Abstract

Investigations of elemental abundances in the ancient and most metal deficient stars are extremely important because they serve as tests of variable nucleosynthesis pathways and can provide critical inferences of the type of stars that lived and died before them. The presence of r-process elements in a handful of carbon-enhanced metal-poor (CEMP-r) stars, which are assumed to be closely connected to the chemical yield from the first stars, is hard to reconcile with standard neutron star mergers. Here we show that the production rate of dynamically assembled compact binaries in high-z nuclear star clusters can attain a sufficient high value to be a potential viable source of heavy r-process material in CEMP-r stars. The predicted frequency of such events in the early Galaxy, much lower than the frequency of Type II supernovae but with significantly higher mass ejected per event, can naturally lead to a high level of scatter of Eu as observed in CEMP-r stars.