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

Abstract

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.

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Fig. 1: Abundance ratios of Mg and Eu as a function of metallicity in J1124+4535 and Galactic halo and disk stars.
Fig. 2: Abundance ratios of Mg and Eu as a function of metallicity in J1124+4535 and other metal-poor stars.
Fig. 3: Abundance ratios as a function of metallicity in J1124+4535 and stars in the classical dwarf galaxy UMi.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

Support for this work was provided by the National Natural Science Foundation of China grant numbers 11890694, 11390371, 11603033, 11573032 and 11773033. This research was supported by JSPS-CAS Joint Research Program. W.A. was supported by JSPS KAKENHI grant number 16H02168. M.N.I. was supported by JSPS KAKENHI grant number 17K14249. This paper includes data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Funding for LAMOST (www.lamost.org) has been provided by the Chinese NDRC. LAMOST is operated and managed by the National Astronomical Observatories, CAS.

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Q.-F.X. is responsible for the data analysis and paper draft. G.Z. proposed and initiated this research subject. W.A. and T.M. obtained the high-resolution spectra. H.-N.L., S.H. and M.N.I. assisted Q.-F.X with the data reduction and interpretation of observed chemical abundances. The manuscript was further revised by Q.-F.X., G.Z. and W.A. with all authors contributing comments and suggestions.

Corresponding authors

Correspondence to Qian-Fan Xing or Gang Zhao.

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Journal peer review information: Nature Astronomy thanks Rana Ezzeddine and Terese Hansen for their contribution to the peer review of this work.

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Xing, Q., Zhao, G., Aoki, W. et al. Evidence for the accretion origin of halo stars with an extreme r-process enhancement. Nat Astron 3, 631–635 (2019). https://doi.org/10.1038/s41550-019-0764-5

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