Stochastic Chemical Evolution of Galactic Subhalos and the Origin of
  r-Process Elements
  Mergers of compact binaries (of a neutron star and another neutron star or a
black hole, NSMs) are suggested to be the promising astrophysical site of the
r-process. While the average coalescence timescale of NSMs appears to be > 100
Myr, most of previous chemical evolution models indicate that the observed
early appearance and large dispersion of [r/Fe] in Galactic halo stars at
[Fe/H] < -2.5 favors shorter coalescence times of 1-10 Myr. We argue that this
is not the case for the models assuming the formation of the Galactic halo from
clustering of subhalos with different star formation histories as suggested by
Ishimaru et al. (2015). We present a stochastic chemical evolution model of the
subhalos, in which the site of the r-process is assumed to be mainly NSMs with
a coalescence timescale of 100 Myr. In view of the scarcity of NSMs, their
occurrence in each subhalo is computed with a Monte Carlo method. Our results
show that the less massive subhalos evolve at lower metallicities and generate
highly r-process-enhanced stars. An assembly of these subhalos leaves behind
the large star-to-star scatters of [r/Fe] in the Galactic halo as observed.
However, the observed scatters of [Sr/Ba] at low metallicities indicate the
presence of an additional site that partially contributes to the enrichment of
light neutron-capture elements such as Sr. The high enhancements of [r/Fe] at
low metallicities found in our low-mass subhalo models also qualitatively
reproduce the abundance signatures of the stars in the recently discovered
ultra-faint dwarf galaxy Reticulum II. Therefore, our results suggest NSMs as
the dominant sources of r-process elements in the Galactic halo.
