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 ($+0.3⩽\left[Eu/Fe\right]⩽+1.0$) r-I and strongly r-process enhanced ($\left[Eu/Fe\right]>+1.0$) r-II stars to include an r-III class, for r-process stars such as J1521−3538, with $\left[Eu/Fe\right]>+2.0$ and $\left[Ba/Eu\right]<-0.5$, or $⩾100$ times the solar ratio of europium to iron. Using cosmochronometry, we estimate J1521−3538 to be $12.5\pm 5Gyr$ and $8.9\pm 5Gyr$, 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.