The R-Process Alliance: 2MASS J22132050–5137385, the Star with the Highest-known r-process Enhancement at [Eu/Fe] = +2.45

We present stellar parameters and chemical abundances of 47 elements detected in the bright (V = 11.63) very metal-poor ([Fe/H] = ‑2.20 ± 0.12) star 2MASS J22132050‑5137385. We observed this star using the Magellan Inamori Kyocera Echelle spectrograph as part of ongoing work by the R-Process Alliance. The spectrum of 2MASS J22132050‑5137385 exhibits unusually strong lines of elements heavier than the iron group, and our analysis reveals that these elements were produced by rapid neutron-capture (r-process) nucleosynthesis. We derive a europium enhancement, [Eu/Fe] = +2.45 ± 0.08, that is higher than any other r-process-enhanced star known at present. This star is only the eighth r-process-enhanced star where both thorium and uranium are detected, and we calculate the age of the r-process material, 13.6 ± 2.6 Gyr, from the radioactive decay of these isotopes. This star contains relatively large enhancements of elements that may be produced as transuranic fission fragments, and we propose a new method using this characteristic to assess the r-process yields and gas dilution in samples of r-process-enhanced stars. Assuming a canonical baryonic minihalo mass of 10⁶ M _⊙ and a 1% metal retention rate, this star formed in a cloud of only ∼600 M _⊙. We conclude that 2MASS J22132050‑5137385 exhibits a high level of r-process enhancement because it formed in an environment where the r-process material was less diluted than average. ^* This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.