Extreme r-process Enhanced Stars at High Metallicity in Fornax*

We present and discuss three extremely rapid neutron-capture (r-)process enhanced stars located in the massive dwarf spheriodal galaxy Fornax. These stars are very unique with an extreme Eu enrichment at high metallicities. They have the largest Eu abundances ever observed in a dwarf galaxy opening new opportunities to further understand the origin of heavy elements formed by the r-process. We derive stellar abundances of Co, Zr, La, Ce, Pr, Nd, Er, and Lu using 1-dimensional, local thermodynamic equilibrium (LTE) codes and model atmospheres in conjunction with state-of-the art yield predictions. We derive Zr in the largest sample of stars (105) known to date in a dwarf galaxy. Accurate stellar abundances combined with a careful assessment of the yield predictions have revealed three metal-rich stars in Fornax showing a pure r-process pattern. We define a new class of stars, namely Eu-stars, as r-II stars (i.e., [Eu/Fe]$>1$) at high metallicities (i.e., $\mathrm{[Fe/H]}\gtrsim -1.5$). The stellar abundance pattern contains Lu, observed for the first time in a dwarf galaxy, and reveals that a late burst of star formation has facilitated extreme r-process enhancement late in the galaxy's history ($<4$ Gyr ago). Owing to the large uncertainties associated with nuclear physics input in the yield predictions, we cannot yet determine the r-process site leading to the three Eu-stars in Fornax. Our results demonstrate that extremely r-rich stars are not only associated to ultra faint low-mass dwarf galaxies, but can be born also in massive dwarf galaxies.