GW170817$-$the first observed neutron star merger and its kilonova:
  implications for the astrophysical site of the r-process
  The first neutron star (NS) merger observed by advanced LIGO and Virgo,
GW170817, and its fireworks of electromagnetic counterparts across the entire
electromagnetic spectrum marked the beginning of multi-messenger astronomy and
astrophysics with gravitational waves. The ultraviolet, optical, and
near-infrared emission was consistent with being powered by the radioactive
decay of nuclei synthesized in the merger ejecta by the rapid neutron capture
process (r-process). Starting from an outline of the inferred properties of
this 'kilonova' emission, I discuss possible astrophysical sites for r-process
nucleosynthesis in NS mergers, arguing that the heaviest r-process elements
synthesized in this event most likely originated in outflows from a post-merger
accretion disk. I compare the inferred properties of r-process element
production in GW170817 to current observational constraints on galactic heavy
r-process nucleosynthesis and discuss challenges merger-only models face in
explaining the r-process content of our galaxy. Based on the observational
properties of GW170817 and recent theoretical progress on r-process
nucleosynthesis in collapsars, I then show how GW170817 points to collapsars as
the dominant source of r-process enrichment in the Milky Way. These rare
core-collapse events arguably better satisfy existing constraints and overcome
problems related to r-process enrichment in various environments that NS
mergers face. Finally, I comment on the universality of the r-process and on
how variations in light r-process elements can be obtained both in NS mergers
and collapsars.
