Collapsars as a major source of r-process elements
  The production of elements by rapid neutron capture (r-process) in
neutron-star mergers is expected theoretically and is supported by
multimessenger observations of gravitational-wave event GW170817: this
production route is in principle sufficient to account for most of the
r-process elements in the Universe. Analysis of the kilonova that accompanied
GW170817 identified delayed outflows from a remnant accretion disk formed
around the newly born black hole as the dominant source of heavy r-process
material from that event. Similar accretion disks are expected to form in
collapsars (the supernova-triggering collapse of rapidly rotating massive
stars), which have previously been speculated to produce r-process elements.
Recent observations of stars rich in such elements in the dwarf galaxy
Reticulum II, as well as the Galactic chemical enrichment of europium relative
to iron over longer timescales, are more consistent with rare supernovae acting
at low stellar metallicities than with neutron-star mergers. Here we report
simulations that show that collapsar accretion disks yield sufficient r-process
elements to explain observed abundances in the Universe. Although these
supernovae are rarer than neutron-star mergers, the larger amount of material
ejected per event compensates for the lower rate of occurrence. We calculate
that collapsars may supply more than 80 per cent of the r-process content of
the Universe.
