Could Failed Supernovae Explain the High r-process Abundances in Some Low Metallicity Stars?

Rapid neutron capture process (r-process) elements have been detected in a large number of metal-poor halo stars. The observed large abundance scatter in these stars suggests that r-process elements have been produced in a site that is rare compared to core-collapse supernovae (CCSNe). Although being rare, neutron star mergers (NSM) alone have difficulties explaining the observations, especially at low metallicities. In this paper, we present a complementary scenario: Using black hole - neutron star mergers (BHNSMs) as additional r-process site. We show that both sites together are able to explain the observed r-process abundances in the Galaxy.