Neutron star mergers and rare core-collapse supernovae as sources of
  r-process enrichment in simulated galaxies
  We use cosmological, magnetohydrodynamical simulations of Milky Way-mass
galaxies from the Auriga project to study their enrichment with rapid neutron
capture (r-process) elements. We implement a variety of enrichment models from
both binary neutron star mergers and rare core-collapse supernovae. We focus on
the abundances of (extremely) metal-poor stars, most of which were formed
during the first ~Gyr of the Universe in external galaxies and later accreted
onto the main galaxy. We find that the majority of metal-poor stars are
r-process enriched in all our enrichment models. Neutron star merger models
result in a median r-process abundance ratio which increases with metallicity,
whereas the median trend in rare core-collapse supernova models is
approximately flat. The scatter in r-process abundance increases for models
with longer delay times or lower rates of r-process producing events. Our
results are nearly perfectly converged, in part due to the mixing of gas
between mesh cells in the simulations. Additionally, different Milky Way-mass
galaxies show only small variation in their respective r-process abundance
ratios. Current (sparse and potentially biased) observations of metal-poor
stars in the Milky Way seem to prefer rare core-collapse supernovae over
neutron star mergers as the dominant source of r-process elements at low
metallicity, but we discuss possible caveats to our models. Dwarf galaxies
which experience a single r-process event early in their history show highly
enhanced r-process abundances at low metallicity, which is seen both in
observations and in our simulations. We also find that the elements produced in
a single event are mixed with ~10^8 Msun of gas relatively quickly,
distributing the r-process elements over a large region.
