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

Nature Astronomy volume 3, pages 631–635 (2019)Cite this article

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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.

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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.

References

  1. Bullock, J. S. & Johnston, K. V. Tracing galaxy formation with stellar halos. I. Methods. Astrophys. J. 635, 931–949 (2005).

    Article  ADS  Google Scholar 

  2. Ibata, R., Gilmore, G. & Irwin, M. J. A dwarf satellite galaxy in Sagittarius. Nature 370, 194–196 (1994).

    Article  ADS  Google Scholar 

  3. Tinsley, B. M. Stellar lifetimes and abundance ratios in chemical evolution. Astrophys. J. 229, 1046–1056 (1979).

    Article  ADS  Google Scholar 

  4. McWilliam, A. Abundance ratios and galactic chemical evolution. Annu. Rev. Astron. Astrophys. 35, 503–556 (1997).

    Article  ADS  Google Scholar 

  5. Ivans, I. I. et al. Chemical substructure in the milky way halo: a new population of old stars. Astrophys. J. 592, 906–934 (2003).

    Article  ADS  Google Scholar 

  6. Caffau, E. et al. X-shooter GTO: evidence for a population of extremely metal-poor, alpha-poor stars. Astron. Astrophys. 560, A15 (2013).

    Article  Google Scholar 

  7. Tolstoy, E., Hill, V. & Tosi, M. Star-formation histories, abundances, and kinematics of dwarf galaxies in the Local Group. Annu. Rev. Astron. Astrophys. 47, 371–425 (2009).

    Article  ADS  Google Scholar 

  8. Letarte, B. et al. A high-resolution VLT/FLAMES study of individual stars in the centre of the Fornax dwarf spheroidal galaxy. Astron. Astrophys. 523, A17 (2010).

    Article  Google Scholar 

  9. Kobayashi, C., Ishigaki, M. N., Tominaga, N. & Nomoto, K. The origin of low [α/Fe] ratios in extremely metal-poor stars. Astrophys. J. 785, L5 (2014).

    Article  ADS  Google Scholar 

  10. Xing, Q. F. & Zhao, G. Mining the Sloan Digital Sky Survey in search of extremely α-poor stars in the Galaxy. Astrophys. J. 790, 33 (2014).

    Article  ADS  Google Scholar 

  11. Xing, Q. F. & Zhao, G. Newly discovered alpha-poor stars in the solar neighborhood. Astrophys. J. 805, 146 (2015).

    Article  ADS  Google Scholar 

  12. Zhao, G. et al. LAMOST spectral survey—an overview. Res. Astron. Astrophys. 12, 723–734 (2012).

    Article  ADS  Google Scholar 

  13. Noguchi, K. et al. High dispersion spectrograph (HDS) for the subaru telescope. Publ. Astron. Soc. Jpn 54, 855–864 (2002).

    Article  ADS  Google Scholar 

  14. Castelli, F., Gratton, R. G. & Kurucz, R. L. Notes on the convection in the ATLAS9 model atmospheres. Astron. Astrophys. 318, 841–869 (1997).

    ADS  Google Scholar 

  15. Arlandini, C. et al. Neutron capture in low-mass asymptotic giant branch stars: cross sections and abundance signatures. Astrophys. J. 525, 886–900 (1999).

    Article  ADS  Google Scholar 

  16. Hansen, T. T. et al. An r-process enhanced star in the dwarf galaxy Tucana III. Astrophys. J. 838, 44 (2017).

    Article  ADS  Google Scholar 

  17. Roederer, I. U. et al. The r-process alliance: a comprehensive abundance analysis of HD 222925, a metal-poor star with an extreme r-process enhancement of [Eu/H] = −0.14. Astrophys. J. 865, 129 (2018).

    Article  ADS  Google Scholar 

  18. Hansen, T. T. et al. The r-process alliance: first release from the southern search for r-process-enhanced stars in the Galactic halo. Astrophys. J. 858, 92 (2018).

    Article  ADS  Google Scholar 

  19. Howes, L. et al. The EMBLA survey—metal-poor stars in the Galactic bulge. Mon. Not. R. Astron. Soc. 460, 884–901 (2016).

    Article  ADS  Google Scholar 

  20. Aoki, W., Honda, S., Sadakane, K. & Arimoto, N. First determination of the actinide thorium abundance for a red giant of the ursa minor dwarf galaxy. Publ. Astron. Soc. Jpn 59, 15–19 (2007).

    Article  ADS  Google Scholar 

  21. Travaglio, C. et al. Galactic evolution of Sr, Y, and Zr: a multiplicity of nucleosynthetic processes. Astrophys. J. 601, 864–884 (2004).

    Article  ADS  Google Scholar 

  22. Ji, A. P., Frebel, A., Chiti, A. & Simon, J. D. R-process enrichment from a single event in an ancient dwarf galaxy. Nature 531, 610–613 (2016).

    Article  ADS  Google Scholar 

  23. Hirai, Y. et al. Early chemo-dynamical evolution of dwarf galaxies deduced from enrichment of r-process elements. Mon. Not. R. Astron. Soc. 466, 2474–2487 (2017).

    Article  ADS  Google Scholar 

  24. Kasen, D. et al. Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event. Nature 551, 80–84 (2017).

    Article  ADS  Google Scholar 

  25. Nishimura, N., Takiwaki, T. & Thielemann, F. K. The r-process nucleosynthesis in the various jet-like explosions of magnetorotational core-collapse supernovae. Astrophys. J. 810, 109 (2015).

    Article  ADS  Google Scholar 

  26. Lindegren, L. et al. Gaia Data Release 2. Astron. Astrophys. 616, A2 (2018).

    Article  Google Scholar 

  27. Bailer-Jones, C. A. L. et al. Estimating distance from parallaxes. IV. Distances to 1.33 billion stars in Gaia Data Release 2. Astron. J. 156, 2 (2018).

    Article  Google Scholar 

  28. Sneden, C. A. Carbon and Nitrogen Abundances in Metal-poor Stars. PhD thesis, Univ. Texas, Austin (1973).

  29. Ivans, I. I. et al. Near-ultraviolet observations of HD 221170: new insights into the nature of r-process-rich stars. Astrophys. J. 645, 613–633 (2006).

    Article  ADS  Google Scholar 

  30. Ishigaki, M. N., Aoki, W. & Chiba, M. Chemical abundances of the Milky Way thick disk and stellar halo. II. Sodium, iron-peak, and neutron-capture elements. Astrophys. J. 771, 67 (2013).

    Article  ADS  Google Scholar 

  31. Lind, K., Asplund, M., Barklem, P. S. & Belyaev, A. K. Non-LTE calculations for neutral Na in late-type stars using improved atomic data. Astron. Astrophys. 528, 103 (2011).

    Article  ADS  Google Scholar 

  32. Xing, Q. F. & Zhao, G. Evidence for accreted component in the Galactic discs. Mon. Not. R. Astron. Soc. 476, 5388–5392 (2018).

    Article  ADS  Google Scholar 

  33. Bovy, J. galpy: a python library for Galactic dynamics. Astrophys. J. Suppl. 216, 2 (2015).

    Article  Google Scholar 

  34. Frebel, A. Stellar archaeology: exploring the Universe with metal-poor stars. Astron. Nachr. 331, 474–488 (2010).

    Article  ADS  Google Scholar 

  35. Roederer, I. U., Hattori, K. & Valluri, M. Kinematics of highly r-process-enhanced field stars: evidence for an accretion origin and detection of several groups from disrupted satellites. Astron. J. 156, 179 (2018).

    Article  ADS  Google Scholar 

  36. Suda, T. et al. The stellar abundances for galactic archaeology (SAGA) database—IV. Compilation of stars in dwarf galaxies. Publ. Astron. Soc. Jpn 69, 76 (2017).

    Article  ADS  Google Scholar 

  37. Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. Annu. Rev. Astron. Astrophys. 47, 481–522 (2009).

    Article  ADS  Google Scholar 

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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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Authors and Affiliations

  1. Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China

    Qian-Fan Xing, Gang Zhao & Hai-Ning Li

  2. National Astronomical Observatory of Japan, Tokyo, Japan

    Wako Aoki & Tadafumi Matsuno

  3. Department of Astronomical Science, School of Physical Sciences, The Graduate University of Advanced Studies (SOKENDAI), Tokyo, Japan

    Wako Aoki & Tadafumi Matsuno

  4. Center for Astronomy, University of Hyogo, Sayo, Japan

    Satoshi Honda

  5. Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kashiwa, Japan

    Miho N. Ishigaki

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  1. Qian-Fan Xing
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Contributions

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.

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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, QF., 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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