NEUTRON STAR MERGERS AS THE ORIGIN OF r-PROCESS ELEMENTS IN THE GALACTIC HALO BASED ON THE SUB-HALO CLUSTERING SCENARIO

Recent hydrodynamical and nucleosynthesis studies have suggested binary mergers (NSMs) of double neutron star (and black-hole-neutron-star) systems as major sites of r-process elements in the Galaxy. 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] similar to -3. To resolve this problem, we examine the role of NSMs in the early Galactic chemical evolution with 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-circle dot and 2 x 10(8) M-circle dot. The 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 metallicities 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, sub-solar [r/Fe] values (observed as [Ba/Fe] similar to -1.5 for [Fe/H] < -3) are explained by the contribution from short-lived (similar to 1 Myr) binaries. Our results indicate that NSMs may have contributed substantially to the r-process element abundances throughout the history of the Galaxy.