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R-process enrichment from a single event in an ancient dwarf galaxy


Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture processes1,2. The main site of production of the r-process elements (such as europium) has been debated for nearly 60 years2. Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae3,4. But evidence from the local Universe favours the idea that r-process production occurs mainly during rare events, such as neutron star mergers5,6. The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-process enrichment in the early Universe7, but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion8 favours continual r-process enrichment in these systems. Furthermore, the universal r-process pattern1,9 has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-process10,11. Reticulum II is one such galaxy12,13,14. The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars15. Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-process pattern beyond barium. The enhancement seen in this ‘r-process galaxy’ is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy11,16,17. This implies that a single, rare event produced the r-process material in Reticulum II. The r-process yield and event rate are incompatible with the source being ordinary core-collapse supernovae18, but consistent with other possible sources, such as neutron star mergers19.

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Figure 1: Spectra of stars in Reticulum II.
Figure 2: Chemical abundances of stars in Reticulum II.


  1. Sneden, C., Cowan, J. J. & Gallino, R. Neutron-capture elements in the early galaxy. Annu. Rev. Astron. Astrophys. 46, 241–288 (2008)

    ADS  CAS  Google Scholar 

  2. Burbidge, E. M., Burbidge, G. R., Fowler, W. A. & Hoyle, F. Synthesis of the elements in stars. Rev. Mod. Phys. 29, 547–650 (1957)

    ADS  Google Scholar 

  3. Qian, Y. Z. Supernovae versus neutron star mergers as the major r-process sources. Astrophys. J. 534, L67–L70 (2000)

    ADS  CAS  PubMed  Google Scholar 

  4. Argast, D., Samland, M., Thielemann, F. K. & Qian, Y. Z. Neutron star mergers versus core-collapse supernovae as dominant r-process sites in the early Galaxy. Astron. Astrophys. 416, 997–1011 (2004)

    ADS  CAS  Google Scholar 

  5. Tanvir, N. R. et al. A ‘kilonova’ associated with the short-duration γ-ray burst GRB 130603B. Nature 500, 547–549 (2013)

    ADS  CAS  PubMed  Google Scholar 

  6. Wallner, A. et al. Abundance of live 244Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis. Nature Commun . 6, 5956 (2015)

    ADS  CAS  Google Scholar 

  7. Tsujimoto, T., Ishigaki, M. N., Shigeyama, T. & Aoki, W. Chemical feature of Eu abundance in the Draco dwarf spheroidal galaxy. Publ. Astron. Soc. Jpn 67, L3 (2015)

    ADS  Google Scholar 

  8. Kirby, E. N., Lanfranchi, G. A., Simon, J. D., Cohen, J. G. & Guhathakurta, P. Multi-element abundance measurements from medium-resolution spectra. III. Metallicity distributions of Milky Way dwarf satellite galaxies. Astrophys. J. 727, 78 (2011)

    ADS  Google Scholar 

  9. Burris, D. L. et al. Neutron-capture elements in the early galaxy: insights from a large sample of metal-poor giants. Astrophys. J. 544, 302–319 (2000)

    ADS  CAS  Google Scholar 

  10. Ji, A. P., Frebel, A. & Bromm, V. Preserving chemical signatures of primordial star formation in the first low-mass stars. Mon. Not. R. Astron. Soc. 454, 659–674 (2015)

    ADS  CAS  Google Scholar 

  11. Frebel, A., Simon, J. D. & Kirby, E. N. Segue 1: an unevolved fossil galaxy from the early Universe. Astrophys. J. 786, 74 (2014)

    ADS  Google Scholar 

  12. Bechtol, K. et al. Eight new Milky Way companions discovered in first-year Dark Energy Survey data. Astrophys. J. 807, 50 (2015)

    ADS  Google Scholar 

  13. Koposov, S. E., Belokurov, V., Torrealba, G. & Evans, N. W. Beasts of the southern wild: discovery of nine ultra faint satellites in the vicinity of the Magellanic Clouds. Astrophys. J. 805, 130 (2015)

    ADS  Google Scholar 

  14. Simon, J. D. et al. Stellar kinematics and metallicities in the ultra-faint dwarf galaxy Reticulum II. Astrophys. J. 808, 95 (2015)

    ADS  Google Scholar 

  15. Roederer, I. U. et al. Detailed chemical abundances in the r-process-rich ultra-faint dwarf galaxy Reticulum 2. Preprint at (2016)

  16. Ji, A. P., Frebel, A., Simon, J. D. & Geha, M. C. High-resolution spectroscopy of extremely metal-poor stars in the least evolved galaxies: Bootes II. Astrophys. J. 817, 41 (2016)

    ADS  Google Scholar 

  17. François, P. et al. Abundance ratios of red giants in low mass ultra faint dwarf spheroidal galaxies. Preprint at arXiv:1510.05401 (2015)

  18. Wanajo, S. The r-process in proto-neutron-star Wind revisited. Astrophys. J. 770, L22 (2013)

    ADS  Google Scholar 

  19. Goriely, S., Bauswein, A. & Janka, H.-T. R-process nucleosynthesis in dynamically ejected matter of neutron star mergers. Astrophys. J. 738, L32 (2011)

    ADS  Google Scholar 

  20. Brown, T. M. et al. The quenching of the ultra-faint dwarf galaxies in the reionization era. Astrophys. J. 796, 91 (2014)

    ADS  CAS  Google Scholar 

  21. Kirby, E. N., Simon, J. D., Geha, M. C., Guhathakurta, P. & Frebel, A. Uncovering extremely metal-poor stars in the Milky Way’s ultrafaint dwarf spheroidal satellite galaxies. Astrophys. J. 685, L43–L46 (2008)

    ADS  CAS  Google Scholar 

  22. Dominik, M. et al. Double compact objects. I. The significance of the common envelope on merger rates. Astrophys. J. 759, 52 (2012)

    ADS  Google Scholar 

  23. Wehmeyer, B., Pignatari, M. & Thielemann, F. K. Galactic evolution of rapid neutron capture process abundances: the inhomogeneous approach. Mon. Not. R. Astron. Soc. 452, 1970–1981 (2015)

    ADS  CAS  Google Scholar 

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

    ADS  CAS  Google Scholar 

  25. Komiya, Y., Yamada, S., Suda, T. & Fujimoto, M. Y. The new model of chemical evolution of r-process elements based on the hierarchical galaxy formation. I. Ba and Eu. Astrophys. J. 783, 132 (2014)

    ADS  Google Scholar 

  26. Shen, S. et al. The history of r-process enrichment in the Milky Way. Astrophys. J. 807, 115 (2015)

    ADS  Google Scholar 

  27. Van de Voort, F., Quataert, E., Hopkins, P. F., Kere, D. & Faucher-Giguere, C. A. Galactic r-process enrichment by neutron star mergers in cosmological simulations of a Milky Way-mass galaxy. Mon. Not. R. Astron. Soc. 447, 140–148 (2014)

    ADS  Google Scholar 

  28. Ishimaru, Y., Wanajo, S. & Prantzos, N. Neutron star mergers as the origin of r-process elements in the galactic halo based on the sub-halo clustering scenario. Astrophys. J. 804, L35 (2015)

    ADS  Google Scholar 

  29. Bland-Hawthorn, J., Sutherland, R. & Webster, D. Ultrafaint dwarf galaxies—the lowest-mass relics from before reionization. Astrophys. J. 807, 154 (2015)

    ADS  Google Scholar 

  30. Frebel, A. & Bromm, V. Chemical signatures of the first galaxies: criteria for one-shot enrichment. Astrophys. J. 759, 115 (2012)

    ADS  Google Scholar 

  31. Walker, M. G. et al. Magellan/M2FS spectroscopy of the Reticulum 2 dwarf spheroidal galaxy. Astrophys. J. 808, 108 (2015)

    ADS  Google Scholar 

  32. Koposov, S. E. et al. Kinematics and chemistry of recently discovered Reticulum 2 and Horologium 1 dwarf galaxies. Astrophys. J. 811, 62 (2015)

    ADS  Google Scholar 

  33. Bernstein, R., Shectman, S. A., Gunnels, S. M., Mochnacki, S. & Athey, A. E. MIKE: a double echelle spectrograph for the Magellan telescopes at Las Campanas Observatory. Proc. SPIE 4841, 1694–1704 (2003)

    ADS  Google Scholar 

  34. Kelson, D. D. Optimal techniques in two-dimensional spectroscopy: background subtraction for the 21st century. Publ. Astron. Soc. Pacif . 115, 688–699 (2003)

    ADS  Google Scholar 

  35. Casey, A. R. A tale of tidal tails in the Milky Way. Preprint at arXiv:1405.5968 (2014)

  36. Frebel, A., Casey, A. R., Jacobson, H. R. & Yu, Q. Deriving stellar effective temperatures of metal-poor stars with the excitation potential method. Astrophys. J. 769, 57 (2013)

    ADS  Google Scholar 

  37. Castelli, F. & Kurucz, R. L. New grids of ATLAS9 model atmospheres. Preprint at arXiv:astro-ph/0405087 (2004)

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

  39. Kim, Y. C., Demarque, P., Yi, S. K. & Alexander, D. R. The Y2 isochrones for α-element enhanced mixtures. Astrophys. J . 143 (Suppl.), 499–511 (2002)

    CAS  Google Scholar 

  40. Hill, V. et al. First stars. I. The extreme r-element rich, iron-poor halo giant CS 31082-001. Implications for the r-process site(s) and radioactive cosmochronology. Astron. Astrophys. 387, 560–579 (2002)

    ADS  CAS  Google Scholar 

  41. 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)

    ADS  CAS  Google Scholar 

  42. Sneden, C., Lawler, J. E., Cowan, J. J., Ivans, I. I. & Den Hartog, E. A. New rare earth element abundance distributions for the Sun and five r-process-rich very metal-poor stars. Astrophys. J . 182 (Suppl.), 80–96 (2009)

    CAS  Google Scholar 

  43. Roederer, I. U. et al. A search for stars of very low metal abundance. VI. Detailed abundances of 313 metal-poor stars. Astrophys. J. 147, 136 10.1088/0004-6256/147/6/136 (2014)

    Google Scholar 

  44. Keeping, E. S. Introduction to Statistical Inference Ch. 8 (Van Nostrand, 1962)

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

    ADS  CAS  Google Scholar 

  46. Greif, T. H., Glover, S. C. O., Bromm, V. & Klessen, R. S. The first galaxies: chemical enrichment, mixing, and star formation. Astrophys. J. 716, 510–520 (2010)

    ADS  CAS  Google Scholar 

  47. McConnachie, A. W. The observed properties of dwarf galaxies in and around the local group. Astrophys. J. 144, 4 10.1088/0004-6256/144/1/4 (2012)

    Google Scholar 

  48. Dotter, A. et al. The Dartmouth stellar evolution database. Astrophys. J . 178 (Suppl.), 89–101 (2008)

    CAS  Google Scholar 

  49. Geha, M. C. et al. The stellar initial mass function of ultra-faint dwarf galaxies: evidence for IMF variations with galactic environment. Astrophys. J. 771, 29 (2013)

    ADS  Google Scholar 

  50. Abadie, J. et al. Topical review: predictions for the rates of compact binary coalescences observable by ground-based gravitational-wave detectors. Class. Quantum Gravity 27, 173001 (2010)

    ADS  Google Scholar 

  51. Bramante, J. & Linden, T. On the r-process enrichment of dwarf spheroidal galaxies. Preprint at arXiv:1601.06784 (2016)

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We gathered data using the 6.5-metre Magellan Clay telescope located at Las Campanas Observatory, Chile. A.P.J. thanks N. Weinberg and P. Schechter for discussions. A.P.J. and A.F. are supported by National Science Foundation (NSF)-CAREER grant AST-1255160. A.F. acknowledges support from the Silverman (1968) Family Career Development Professorship. J.D.S. acknowledges support from NSF grant AST-1108811. This work made use of NASA’s Astrophysics Data System Bibliographic Services and the open-source Python libraries numpy, scipy, matplotlib, statsmodels, pandas, seaborn, and astropy. We also used data products originally obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES) collaboration. The DES Projects have been funded by the DOE and the NSF (USA), MISE (Spain), STFC (UK), HEFCE (UK), NCSA (UIUC), KICP (Univ. Chicago), CCAPP (Ohio State), MIFPA (Texas A&M), CNPQ, FAPERJ and FINEP (Brazil), MINECO (Spain), DFG (Germany) and the collaborating institutions in the Dark Energy Survey, which are Argonne Lab, UC Santa Cruz, University of Cambridge, CIEMAT-Madrid, University of Chicago, University College London, DES-Brazil Consortium, University of Edinburgh, ETH Zürich, Fermilab, University of Illinois, ICE (IEEC-CSIC), IFAE Barcelona, Lawrence Berkeley Lab, LMU München and the associated Excellence Cluster Universe, University of Michigan, NOAO, University of Nottingham, Ohio State University, University of Pennsylvania, University of Portsmouth, SLAC National Lab, Stanford University, University of Sussex, and Texas A&M University.

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Authors and Affiliations



A.P.J. took the observations and led the analysis and paper writing; A.F. and A.C. assisted with the observations; A.F. and J.D.S. contributed to the analysis; all authors contributed to writing the paper.

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Correspondence to Alexander P. Ji.

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Extended data figures and tables

Extended Data Figure 1 Properties of Reticulum II member stars.

a, Coordinates of member stars, in right ascension (RA) and declination (DEC) at the standard epoch (J2000)14. Stars selected for observation with high-resolution spectroscopy are highlighted with large coloured circles, while other members are shown in black. b, Colour–magnitude diagram based on Dark Energy Survey photometry14; g and r are the stars’ magnitudes in two different filters.

Extended Data Table 1 Stellar-parameter uncertainties
Extended Data Table 2 Abundances of neutron-capture elements

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Ji, A., Frebel, A., Chiti, A. et al. R-process enrichment from a single event in an ancient dwarf galaxy. Nature 531, 610–613 (2016).

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