Short-lived $^{244}$Pu Points to Compact Binary Mergers as Sites for Heavy r-process Nucleosynthesis

Abstract

Measurements of the radioactive $^{244}$Pu abundances can break the degeneracy between high-rate/low-yield and low-rate/high-yield scenarios for the production of heavy $r$-process elements. The first corresponds to production by core collapse supernovae (cc-SNe) while the latter corresponds to production by e.g. compact binary mergers. The estimated $^{244}$Pu abundance in the current interstellar medium inferred from deep-sea measurements (Wallner et al. 2015) is significantly lower than that corresponding Early Solar System abundances (Turner et al 2007). We estimate the expected median value of the $^{244}$Pu abundances and fluctuations around this value in both models. We show that while the current and Early Solar System abundances are naturally explained within the low-rate/high-yield (e.g. merger) scenario, they are incompatible with the high-rate/low-yield (cc-SNe) model. The inferred event rate remarkably agrees with compact binary merger rates estimated from Galactic neutron star binaries and from short gamma-ray bursts. Furthermore, the ejected mass of $r$-process elements per event agrees with both theoretical and observational macronova/kilonova estimates.