THE NEW MODEL OF CHEMICAL EVOLUTION OF r-PROCESS ELEMENTS BASED ON THE HIERARCHICAL GALAXY FORMATION. I. Ba AND Eu

We investigate the chemical enrichment of r-process elements in the early evolutionary stages of the Milky Way halo within the framework of hierarchical galaxy formation using a semi-analytic merger tree. In this paper, we focus on heavy r-process elements, Ba and Eu, of extremely metal-poor (EMP) stars and give constraints on their astronomical sites. Our models take into account changes of the surface abundances of EMP stars by the accretion of interstellar medium (ISM). We also consider metal-enrichment of intergalactic medium by galactic winds and the resultant pre-enrichment of proto-galaxies. The trend and scatter of the observed r-process abundances are well reproduced by our hierarchical model with similar to 10% of core-collapse supernovae in low-mass end (similar to 10 M-circle dot) as a dominant r-process source and the star formation efficiency of similar to 10(-10) yr(-1). For neutron star mergers as an r-process source, their coalescence timescale has to be similar to 10(7) yr, and the event rates similar to 100 times larger than currently observed in the Galaxy. We find that the accretion of ISM is a dominant source of r-process elements for stars with [Ba/ H] <-3.5. In this model, a majority of stars at [Fe/H] <-3 are formed without r-process elements, but their surfaces are polluted by the ISM accretion. The pre-enrichment affects similar to 4% of proto-galaxies, and yet, is surpassed by the ISM accretion in the surface of EMP stars.