The R-Process Alliance: A Very Metal-poor, Extremely r-process-enhanced Star with [Eu/Fe] = + 2.2, and the Class of r-III Stars

We report the discovery of J1521−3538, a bright ( V = 12.2), very metal-poor ([Fe/H] = −2.8) strongly r -process-enhanced field horizontal branch star, based on a high-resolution, high signal-to-noise Magellan/MIKE spectrum. J1521−3538 shows the largest r -process element overabundance in any known r -process-enhanced star, with [Eu/Fe] = +2.2, and its chemical abundances of 22 neutron-capture elements closely match the scaled solar r -process pattern. J1521−3538 is also one of few known carbon-enhanced metal-poor stars with r -process enhancement (CEMP- r stars), as found after correcting the measured C abundance for the star’s evolutionary status. We propose to extend the existing classification of moderately enhanced ( ) r -I and strongly r -process enhanced ( ) r -II stars to include an r -III class, for r -process stars such as J1521−3538, with and , or times the solar ratio of europium to iron. Using cosmochronometry, we estimate J1521−3538 to be and , using two different sets of initial production ratios. These ages are based on measurements of the Th line at 4019 Å and other r -process element abundances. This is broadly consistent with the old age of a low-mass, metal-poor field red horizontal branch star. J1521−3538 likely originated in a low-mass dwarf galaxy that was later accreted by the Milky Way, as evidenced by its highly eccentric orbit.