Evidence for the accretion origin of halo stars with an extreme
  r-process enhancement
  Small stellar systems like dwarf galaxies are suggested to be the main
building blocks of our Galaxy by numerical simulations in Lambda CDM models.
The existence of star streams like Sagittarius tidal stream indicates that
dwarf galaxies play a role in the formation of the Milky Way. However, it is
unclear how many and what kind of stars in our Galaxy are originated from
satellite dwarf galaxies, which could be constrained by chemical abundances of
metal-poor stars. Here we report on the discovery of a metal-poor star with an
extreme r-process enhancement and alpha-element deficiency. In this star, the
abundance ratio of the r-process element Eu with respect to Fe is more than one
order of magnitude higher than the Sun and the metallicity is 1/20 of the solar
one. Such kind of stars have been found in present-day dwarf galaxies,
providing the clearest chemical signature of past accretion events. The long
timescale of chemical evolution of the host dwarf galaxy expected from the
abundance of alpha element with respect to Fe suggests that the accretion
occurred in a relatively late phase compared to most of the accretions that
formed the bulk of the Milky Way halo.
