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 10 ^6 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.