Natal Kicks and Time Delays in Merging Neutron Star Binaries - Implications for r-process nucleosynthesis in Ultra Faint Dwarfs and in the Milky Way

TL;DR

This paper investigates how neutron star binary properties influence early heavy r-process element production in ultra faint dwarf galaxies and the Milky Way, addressing observed element abundances despite potential ejection from small galaxies.

Contribution

It demonstrates that a subset of neutron star binaries with low proper motion can merge quickly enough to explain early r-process enrichment in small galaxies and the Milky Way.

Findings

A significant fraction of neutron star binaries have low proper motion.

Many binaries merge within 100-300 million years.

Rapid mergers can account for early heavy r-process element presence.

Abstract

Merging neutron star binaries are prime candidate sources for heavy r-process nucleosynthesis. The amount of heavy r-process material is consistent with the mass ejection and rates of mergers, and abundances of relic radioactive materials suggest that heavy r-process material is produced in rare events. Observations of possible macronovae provide further support for this model. Still, some concerns remain. One is the observation of heavy r-process elements in Ultra Faint Dwarf (UFD) galaxies. The escape velocities from UFDs are so small that the natal kicks, taking place at neutron stars birth, might eject such binaries from UFDs. Furthermore the old stellar populations of UFDs requires that r-process nucleosynthesis must have taken place very early on, while it may take several Gyr for compact binaries to merge. This last problem arises also within the Milky Way where heavy r-process materials has been observed in some low metallicity stars. We show here that since a significant fraction of neutron star binaries form with a very small proper motion those won't be ejected even from a UFD. Furthermore, a significant fraction of these binaries merge within a few hundred Myr and in some cases the merger time is even shorter than a hundred Myr. This population of "rapid mergers" explains the appearance of heavy r-process material in both UFDs and in the Milky Way.