Original article can be found at: http://www.nature.com/nature/index.html--Copyright Nature Publishing Group --DOI : 10.1038/nature0345

We revisit the observed frequencies of Carbon-Enhanced Metal-Poor (CEMP) stars as a function of the metallicity in the Galaxy, using data from the literature with available high-resolution spectroscopy. Our analysis excludes stars exhibiting clear over-abundances of neutron-capture elements, and takes into account the expected depletion of surface carbon abundance that occurs due to CN processing on the upper red-giant branch. This allows for the recovery of the initial carbon abundance of these stars, and thus for an accurate assessment of the frequencies of carbon-enhanced stars. The correction procedure we develope is based on stellar-evolution models, and depends on the surface gravity, log g , of a given star. Our analysis indicates that, for stars with [Fe/H]≤ −2.0, 20% exhibit [C/Fe]≥ +0.7. This fraction increases to 43% for [Fe/H]≤ −3.0 and 81% for [Fe/H]≤ −4.0, which is higher than have been previously inferred without taking the carbon-abundance correction into account. These CEMP-star frequencies provide important inputs for Galactic and stellar chemical-evolution models, as they constrain the evolution of carbon at early times and the possible formation channels for the CEMP-no stars. We also have developed a public online tool with which carbon corrections using our procedure can be easily obtained.

The elements heavier than zinc are synthesized through the (r)apid and (s)low neutroncapture processes 1,2 . The primary astrophysical production site of the r-process elements (such as europium) has been debated for nearly 60 years 2 . Chemical abundance trends of old Galactic halo stars initially suggested continual r-process production from sources like core-collapse supernovae 3,4 , but evidence in the local universe favored r-process production primarily from rare events like neutron star mergers 5,6 . The appearance of a europium abundance plateau in some dwarf spheroidal galaxies was suggested as evidence for rare rprocess enrichment in the early universe 7 , but only under the assumption of no gas accretion into the dwarf galaxies. Invoking cosmologically motivated gas accretion 8 actually favors continual r-process enrichment in those systems. Furthermore, the universal r-process pattern 1,9 has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultra-faint dwarf galaxies assembled shortly after the formation of the first stars and are ideal systems to study nucleosynthesis processes such as the r-process 10,11 . Reticulum II is a recently discovered ultra-faint dwarf galaxy [12][13][14] . Like other such galaxies, the abundances of non-neutron-capture elements are similar to those of other old stars 15 . Here we report that seven of nine stars in Reticulum II observed with high-resolution spectroscopy show strong enhancements in heavy neutroncapture elements with abundances that exactly follow the universal r-process pattern above barium. The enhancement in this "r-process galaxy" is 2-3 orders of magnitude higher than what is seen in any other ultra-faint dwarf galaxy 11,16,17 . This implies that a single rare event produced the r-process material in Reticulum II, whether or not gas accretion was significant in ultra-faint dwarf galaxies. The r-process yield and event rate is incompatible with ordinary core-collapse supernova 18 but consistent with other possible sites, such as neutron star mergers 19 .Ultra-faint dwarfs (UFDs) are small galaxies that orbit the Milky Way and have been discovered by deep wide-area sky surveys 12,13 . Although physically close to us, they are also relics from the era of the first stars and galaxies and thus an ideal place to investigate the first metal enrichment events in the universe 10 . Observations of UFDs provide evidence that they form all their stars within 1-3 Gyr of the Big Bang 20 , their stars contain very small amounts of elements heavier than helium ("metals") 21 , and they are enriched by the metal output of only a few generations of stars 11,20 . The chemical abundances of light elements (less heavy than iron) suggested that corecollapse supernovae were the primary metal sources in these systems 11,16,17 . This conclusion was supported by unusually low neutron-capture element abundances that are consistent with small amounts of neutron-capture element production associated with massive star evolution...