The formation of heavy elements: supernovae vs. mergers of neutron stars and black holes?

Chemical elements heavier than iron are widely accepted to form through rapid neutron capture reactions. Two sites have been identified where such r-processes can potentially take place: supernova explosions of massive (single) stars and mergers of compact objects (neutron stars and/or black holes) in binary stars. Based on a comparison with observational europium abundances, the ability of the latter scenario to produce such elements soon enough after the birth of the Milky Way has been challenged. However, new observations of short period double neutron stars and improved models for the evolution of binary stars revived compact object mergers as sources of heavy elements. This has allowed to implement this scenario in the detailed population number synthesis code for binary evolution of the Vrije Universiteit Brussel, with very promising results.

There is however great room for adaptation of the evolutionary results to ever improving physical models and updated yields of chemical elements. Theoretical predictions made by the code must also continuously be tested against an increasing number of spectroscopic observations, some of the newest even allowing to extend the investigation to other galaxies than our own. Very recent results might even cast doubt on the possibility of heavy element formation in single stars, perhaps leaving the challenge for the compact object merger scenario to explain all heavy elements observed in the universe.