Evidence for the accretion origin of halo stars with an extreme r-process enhancement

Small stellar systems such as dwarf galaxies are suggested to be the main building blocks of our Galaxy according to numerical simulations1 in Λ cold dark matter models and observational support for this hypothesis comes from the existence of stellar streams such as the Sagittarius tidal stream2. However, it is unclear how many and what kind of stars in our Galaxy originate from satellite dwarf galaxies, something that could be constrained by analysing chemical abundances of metal-poor stars. Here we report on the discovery of a metal-poor star with an extreme r-process enhancement and α-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 larger than the Sun and the metallicity is 1/20 of solar metallicity. Stars like this one have been found in present-day dwarf galaxies, providing the clearest chemical signature of past accretion events onto the Milky Way. The long timescale of chemical evolution of the host dwarf galaxy expected from the abundance of α-elements with respect to Fe suggests that the accretion occurred in a relatively late phase compared with most of the accretions that formed the bulk of the Milky Way halo