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.Comment: Accepted for publication in Ap
