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 matter ( ISM ) .
We also consider metal-enrichment of intergalactic medium ( IGM ) 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 \sim 10 \% of core-collapse supernovae in low-mass end ( \sim 10 M _ { \odot } ) as a dominant r -process source and the star formation efficiency of \sim 10 ^ { -10 } \hbox { yr } ^ { -1 } .
For neutron star mergers as an r -process source , their coalescence timescale has to be \sim 10 ^ { 7 } yrs , and the event rates \sim 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 \sim 4 \% of proto-galaxies , and yet , is surpassed by the ISM accretion in the surface of EMP stars .