Collapsars as a major source of r-process elements.

Abstract

The production of elements by rapid neutron capture (r-process) in neutron-star mergers is expected theoretically and is supported by multimessenger observations^1-3 of gravitational-wave event GW170817: this production route is in principle sufficient to account for most of the r-process elements in the Universe⁴. Analysis of the kilonova that accompanied GW170817 identified^5,6 delayed outflows from a remnant accretion disk formed around the newly born black hole^7-10 as the dominant source of heavy r-process material from that event^9,11. 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^12,13. Recent observations of stars rich in such elements in the dwarf galaxy Reticulum II¹⁴, as well as the Galactic chemical enrichment of europium relative to iron over longer timescales^15,16, 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.