Neutron Star Mergers as the Origin of r-Process Elements in the Galactic
  Halo Based on the Sub-halo Clustering Scenario
  Binary mergers (NSMs) of double neutron star (and black hole-neutron star)
systems are suggested to be major sites of r-process elements in the Galaxy by
recent hydrodynamical and nucleosynthesis studies. It has been pointed out,
however, that the estimated long lifetimes of neutron star binaries are in
conflict with the presence of r-process-enhanced halo stars at metallicities as
low as [Fe/H] ~ -3. To resolve this problem, we examine the role of NSMs in the
early Galactic chemical evolution on the assumption that the Galactic halo was
formed from merging sub-halos. We present simple models for the chemical
evolution of sub-halos with total final stellar masses between 10^4 M_solar and
2 x 10^8 M_solar. Typical lifetimes of compact binaries are assumed to be 100
Myr (for 95% of their population) and 1 Myr (for 5%), according to recent
binary population synthesis studies. The resulting metallcities of sub-halos
and their ensemble are consistent with the observed mass-metallicity relation
of dwarf galaxies in the Local Group, and the metallicity distribution of the
Galactic halo, respectively. We find that the r-process abundance ratios [r/Fe]
start increasing at [Fe/H] <= -3 if the star formation efficiencies are smaller
for less massive sub-halos. In addition, the sub-solar [r/Fe] values (observed
as [Ba/Fe] ~ -1.5 for [Fe/H] < -3) are explained by the contribution from the
short-lived (~1 Myr) binaries. Our results indicate that NSMs may have a
substantial contribution to the r-process element abundances throughout the
Galactic history.
