On Neutron Star Mergers as the Source of r-process Enhanced Metal Poor
  Stars in the Milky Way
  We model the history of Galactic r-process enrichment using high-redshift,
high-resolution zoom cosmological simulations of a Milky Way (MW) type halo. We
assume that all r-process sources are neutron star mergers (NSMs) with a power
law delay time distribution. We model the time to mix pollutants at subgrid
scales, which allows us to to better compute the properties of metal poor (MP)
and carbon enhanced metal poor (CEMP) stars, along with statistics of their
r-process enhanced subclasses. Our simulations underpredict the cumulative
ratios of r-process enhanced MP and CEMP stars (MP-r, CEMP-r) over MP and CEMP
stars by about one order of magnitude, even when the minimum coalescence time
of the double neutron stars ($t_{\rm min}$) is set to 1 Myr. No r-process
enhanced stars form if $t_{\rm min}=100$ Myr. Our results show that even when
we adopt the r-process yield estimates observed in GW170817, NSMs by themselves
can only explain the observed frequency of r-process enhanced stars if either
the birth rate of double neutron stars per unit mass of stars is boosted to
$\approx10^{-4} M_\odot^{-1}$ or the Europium yield of each NSM event is
boosted to $\approx 10^{-4} M_{\odot}$.
