r-process Enrichment in the Galactic Halo Characterized by Nucleosynthesis Variation in the Ejecta of Coalescing Neutron Star Binaries

A large star-to-star variation in the abundances of r-process elements, as seen in the [Eu/Fe] ratio for Galactic halo stars, is a prominent feature that is distinguishable from other heavy elements. It is, in part, caused by the presence of highly r-process-enriched stars, classified as r-II stars ([Eu/Fe] >= + 1). In parallel, halo stars show that the ratio of a light r-process element (Y) to Eu is tightly correlated with [Eu/Fe], giving the lowest [Y/Eu] ratio that levels off at r-II stars. On the other hand, recent hydrodynamical simulations of coalescing double neutron stars (cNSNSs) have suggested that r-process sites may be separated into two classes providing different electron-fraction distributions: tidally driven dynamical ejecta and (dynamical or postmerger) nontidal ejecta. Here, we show that a widely spanning feature of [Eu/Fe] can be reproduced by models that consider the different masses of tidally driven dynamical ejecta from both cNSNSs and coalescing black hole/neutron star binaries (cBHNSs). In addition, the observed [Y/Eu] trend is explained by the combined nucleosynthesis in two kinds of ejecta with varying mass asymmetry in double NS systems. Our scenario suggests that massive tidally driven dynamical ejecta accompanied by massive nontidal part from cNSNSs or cBHNSs could alone accommodate r-II abundances, including an actinide boost in some cases. The event rate for cNSNSs estimated from our study agrees with the latest result of similar to 1000 (90% confidence interval of 110-3840) Gpc(-3)yr(-1) by gravitational-wave detection, and a few events per Gpc(3) per year of cBHNSs associated with r-process production are predicted to emerge.