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] . 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 and . 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] if the star formation efficiencies are smaller for less-massive sub-halos. In addition, sub-solar [ r /Fe] values (observed as [Ba/Fe] for [Fe/H] ) are explained by the contribution from short-lived (∼1 Myr) binaries. Our results indicate that NSMs may have contributed substantially to the r -process element abundances throughout the history of the Galaxy.