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

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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.

At a glance


  1. Spectra of stars in Reticulum II.
    Figure 1: Spectra of stars in Reticulum II.

    a, The spectral region around the europium absorption line (412.9 nm) for the nine brightest stars in Reticulum II, identified by the Dark Energy Survey (DES). Absorption is clearly present in seven of the nine Ret II spectra (black lines), including those with modest signal-to-noise ratios. Thin red lines show synthesized fits to the absorption lines (dashed red lines show upper limits). For comparison, dotted green lines show synthesized spectra for each individual star using typical limits found in other UFDs ([Eu/H] = –2.0). Also shown is HE1523-0901, one of the most r-process-enhanced halo stars known1. b, As for a, but showing the region around the barium absorption line (455.4 nm) (the dotted green line shows [Ba/H] = –4.0).

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

    a, [Ba/H] and [Fe/H] abundances in stars from Ret II (red points), in halo stars24 (grey points), and in other UFDs (coloured points; see references within refs 16, 17). The orange and brown vertical bars indicate the abundance ranges that would be expected following a neutron star merger and in a core-collapse supernova, respectively. The dotted black lines show constant [Ba/Fe] abundances. Arrows denote upper limits. Error bars represent 1σ (see Extended Data Table 1 and Methods). b, As in a, but for [Eu/H] abundances. c, Abundance patterns beyond barium for the four brightest europium-enhanced stars in Ret II (black dots; see Extended Data Table 2), compared with the solar r-process and s-process patterns9 (purple and yellow lines, respectively). Solar patterns are scaled to stellar barium abundance. Stars are offset from each other by multiples of five.

  3. Properties of Reticulum II member stars.
    Extended Data Fig. 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.


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


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Author information


  1. Department of Physics & Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • Alexander P. Ji,
    • Anna Frebel &
    • Anirudh Chiti
  2. Joint Institute for Nuclear Astrophysics, Center for the Evolution of the Elements, East Lansing, Minnesota 48824, USA

    • Alexander P. Ji &
    • Anna Frebel
  3. Observatories of the Carnegie Institution of Washington, Pasadena, California 91101, USA

    • Joshua D. Simon


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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The authors declare no competing financial interests.

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Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: Properties of Reticulum II member stars. (58 KB)

    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 Tables

  1. Extended Data Table 1: Stellar-parameter uncertainties (242 KB)
  2. Extended Data Table 2: Abundances of neutron-capture elements (192 KB)

Additional data