The Origin of r-Process Elements in the Milky Way
  Some of the heavy elements, such as gold and europium (Eu), are almost
exclusively formed by the rapid neutron capture process (r-process). However,
it is still unclear which astrophysical site between core-collapse supernovae
and neutron star - neutron star (NS-NS) mergers produced most of the r-process
elements in the universe. Galactic chemical evolution (GCE) models can test
these scenarios by quantifying the frequency and yields required to reproduce
the amount of europium (Eu) observed in galaxies. Although NS-NS mergers have
become popular candidates, their required frequency (or rate) needs to be
consistent with that obtained from gravitational wave measurements. Here we
address the first NS-NS merger detected by LIGO/Virgo (GW170817) and its
associated Gamma-ray burst and analyze their implication on the origin of
r-process elements. The range of NS-NS merger rate densities of 320-4740
Gpc$^{-3}$ yr$^{-1}$ provided by LIGO/Virgo is remarkably consistent with the
range required by GCE to explain the Eu abundances in the Milky Way with NS-NS
mergers, assuming the solar r-process abundance pattern for the ejecta. Under
the same assumption, this event has produced about 1-5 Earth masses of Eu, and
3-13 Earth masses of gold. When using theoretical calculations to derive Eu
yields, constraining the role of NS-NS mergers becomes more challenging because
of nuclear astrophysics uncertainties. This is the first study that directly
combines nuclear physics uncertainties with GCE calculations. If GW170817 is a
representative event, NS-NS mergers can produce Eu in sufficient amounts and
are likely to be the main r-process site.
