NATAL KICKS AND TIME DELAYS IN MERGING NEUTRON STAR BINARIES: IMPLICATIONS FOR r-PROCESS NUCLEOSYNTHESIS IN ULTRA-FAINT DWARFS AND IN THE MILKY WAY

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 require 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 have been observed in some low-metallicity stars. We show here that of neutron star binaries form with a sufficiently small proper motion to remain bound even in a UFD. Furthermore, approximately 90% of double neutron stars with an initial separation of 10 11 cm merge within 300 Myr and merge in less than 100 Myr. This population of “rapid mergers” explains the appearance of heavy r -process material in both UFDs and in the early Milky Way.