r-process enrichment of ultra-faint dwarf galaxies by fast merging double neutron stars

TL;DR

This paper proposes that fast-merging double neutron star systems with large natal kicks can explain the observed r-process element enrichment in ultra-faint dwarf galaxies, challenging previous models.

Contribution

It introduces a new model where rapidly merging DNSs with high natal kicks are responsible for r-process enrichment in UFDs, aligning with observational data.

Findings

Fast-merging DNSs can enrich UFDs with r-process elements.

DNSs with large natal kicks and short merging times are consistent with UFD enrichment.

These DNSs often have highly eccentric orbits or form with short separations.

Abstract

The recent aLIGO/aVirgo discovery of gravitational waves from the neutron star merger (NSM) GW170817 and the follow up kilonova observations have shown that NSMs produce copious amount of r-process material. However, it is difficult to reconcile the large natal kicks and long average merging times of Double Neutron Stars (DNSs), with the levels of r-process enrichment seen in ultra faint dwarf (UFD) galaxies such as Reticulum II and Tucana III. Assuming that such dwarf systems have lost a significant fraction of their stellar mass through tidal stripping, we conclude that contrary to most current models, it is the DNSs with rather large natal kicks but very short merging timescales that can enrich UFD-type galaxies. These binaries are either on highly eccentric orbits, or form with very short separations due to an additional mass-transfer between the first-born neutron star and a naked helium star, progenitor of the second-born neutron star. These DNSs are born with a frequency that agrees with the statistics of the r-process UFDs, and merge well within the virial radius of their host halos, therefore contributing significantly to their r-process enrichment.