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Small stellar systems like dwarf galaxies are suggested to be the mainbuilding blocks of our Galaxy by numerical simulations in Lambda CDM models.The existence of star streams like Sagittarius tidal stream indicates thatdwarf galaxies play a role in the formation of the Milky Way. However, it isunclear how many and what kind of stars in our Galaxy are originated fromsatellite dwarf galaxies, which could be constrained by chemical abundances ofmetal-poor stars. Here we report on the discovery of a metal-poor star with anextreme r-process enhancement and alpha-element deficiency. In this star, theabundance ratio of the r-process element Eu with respect to Fe is more than oneorder of magnitude higher than the Sun and the metallicity is 1/20 of the solarone. Such kind of stars have been found in present-day dwarf galaxies,providing the clearest chemical signature of past accretion events. The longtimescale of chemical evolution of the host dwarf galaxy expected from theabundance of alpha element with respect to Fe suggests that the accretionoccurred in a relatively late phase compared to most of the accretions thatformed the bulk of the Milky Way halo.

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