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) non-tidal 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 non-tidal 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 ~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.
