The R-Process Alliance: Discovery of the First Metal-poor Star with a Combined r- and s-process Element Signature

We present a high-resolution ( R ∼ 35,000), high signal-to-noise ratio (S/N > 200) Magellan/MIKE spectrum of the star RAVE J094921.8−161722, a bright ( V = 11.3) metal-poor red giant star with [Fe/H] = −2.2, identified as a carbon-enhanced metal-poor (CEMP) star from the RAVE survey. We report its detailed chemical abundance signature of light fusion elements and heavy neutron-capture elements. We find J0949−1617 to be a CEMP star with s -process enhancement that must have formed from gas enriched by a prior r -process event. Light neutron-capture elements follow a low-metallicity s -process pattern, while the heavier neutron-capture elements above Eu follow an r -process pattern. The Pb abundance is high, in line with an s -process origin. Thorium is also detected, as expected from an r -process origin, as Th is not produced in the s -process. We employ nucleosynthesis model predictions that take an initial r -process enhancement into account, and then determine the mass transfer of carbon and s -process material from a putative more massive companion onto the observed star. The resulting abundances agree well with the observed pattern. We conclude that J0949−1617 is the first bonafide CEMP- r + s star identified. This class of objects has previously been suggested to explain stars with neutron-capture element patterns that originate from neither the r - nor the s -process alone. We speculate that J0949−1617 formed in an environment similar to those of ultra-faint dwarf galaxies like Tucana III and Reticulum II, which were enriched in r -process elements by one or multiple neutron star mergers at the earliest times.