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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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.