Using failed supernovae to constrain the Galactic r-process element
  production
  Rapid neutron capture process (r-process) elements have been detected in a
large fraction of metal-poor halo stars, with abundances relative to iron (Fe)
that vary by over two orders of magnitude. This scatter is reduced to less than
a factor of 3 in younger Galactic disc stars. The large scatter of r-process
elements in the early Galaxy suggests that the r-process is made by rare
events, like compact binary mergers and rare sub-classes of supernovae.
Although being rare, neutron star mergers alone have difficulties to explain
the observed enhancement of r-process elements in the lowest metallicity stars
compared to Fe. The supernovae producing the two neutron stars already provide
a substantial Fe abundance where the r-process ejecta from the merger would be
injected. In this work we investigate another complementary scenario, where the
r-process occurs in neutron star-black hole mergers in addition to neutron star
mergers. Neutron star-black hole mergers would eject similar amounts of
r-process matter as neutron star mergers, but only the neutron star progenitor
would have produced Fe. Furthermore, a reduced efficiency of Fe production from
single stars significantly alters the age-metallicity relation, which shifts
the onset of r-process production to lower metallicities. We use the
high-resolution [(20 pc)3/cell] inhomogeneous chemical evolution tool `ICE' to
study the outcomes of these effects. In our simulations, an adequate
combination of neutron star mergers and neutron star-black hole mergers
qualitatively reproduces the observed r-process abundances in the Galaxy.
