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

## TL;DR

This paper reports the discovery of a metal-poor star with extreme r-process element enhancement and alpha-element deficiency, providing evidence that some halo stars originated from accreted dwarf galaxies, especially in late accretion events.

## Contribution

It presents the first chemical signature linking an individual halo star to an accreted dwarf galaxy with extreme r-process enrichment.

## Key findings

- Discovery of a star with high Eu/Fe ratio and low metallicity
- Chemical signatures indicating origin from a dwarf galaxy
- Evidence of late accretion event in the Milky Way halo

## Abstract

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.