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 ] \sim - 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 _ { \odot } and 2 \times 10 ^ { 8 } M _ { \odot } .
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 ] \leq - 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 ] \sim - 1.5 for [ Fe/H ] < -3 ) are explained by the contribution from the short-lived ( \sim 1 Myr ) binaries .
Our results indicate that NSMs may have a substantial contribution to the r -process element abundances throughout the Galactic history .