We want to thank all our collaborators with whom we investigated r-process sites and conditions in the past, as well as everybody with whom we had enlightening discussions. Magneto-rotational supernovae, resulting in neutron stars with extremely strong magnetic fields (magnetars), are the most probable candidates for this site.
While neutron star mergers are a clear source of heavy r-process elements, observations of low-metallicity stars indicate the existence of an additional r-process source related to massive (and therefore fast evolving) stars. These must occur with a frequency being smaller by about a factor of 1/100 compared to regular CCSNe, thus excluding the latter as a major source. The early “chemical” evolution of galaxies as well as recent additions of radioactive species to the solar system require to attribute the origin of the heavy r-process elements to very rare events. We review the required nuclear physics input and address the sites which have been suggested so far (among others): core-collapse supernovae (CCSNe), compact binary mergers, and magneto-rotational (MHD-jet) supernovae/magnetars.
However, the astrophysical site where it occurs is still highly uncertain. Despite all remaining uncertainties in nuclear properties far from stability, the r-process is reasonably well understood in terms of its nuclear reaction flow and necessary environment conditions (neutron densities and temperatures). The rapid neutron-capture process (r-process) is responsible for making about half of all heavy elements beyond Fe and is the only source of elements beyond Pb and Bi.
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