Enrichment history of r-process elements in the Galactic halo and dwarf galaxies

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Rapid neutron-capture process (r-process) produces most elements heavier than iron. Recent nucleosynthesis studies have pointed out serious difficulties for supernovae as a candidate of the r-process site. The other strong candidate is a neutron star merger. However, it has been suggested that neutron star mergers could not explain observed enhancement of r-process elements in metal-poor stars because the timescale of merger is regarded to be too long as 100 Myrs. In this study, using a new chemical evolution model, we have shown that neutron star mergers could reproduce observational abundances of metal-poor stars if the Galactic halo were formed from clusterings of sub-halos with various star formation histories. This result strongly supports neutron star mergers as the r-process site.