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We report the discovery of a new actinide-boost star, 2MASSJ09544277+5246414, originally identified as a very bright (V = 10.1), extremelymetal-poor ([Fe/H] = -2.99) K giant in the LAMOST survey, and found to behighly r-process-enhanced (r-II; [Eu/Fe]= +1.28]), during the snapshot phase ofthe R-Process Alliance (RPA). Based on a high S/N, high-resolution spectrumobtained with the Harlan J. Smith 2.7-m telescope, this star is the firstconfirmed actinide-boost star found by RPA efforts. With an enhancement of[Th/Eu] = +0.37, 2MASS J09544277+5246414 is also the most actinide-enhancedr-II star yet discovered, and only the sixth metal-poor star with a measureduranium abundance ([U/Fe] = +1.40). Using the Th/U chronometer, we estimate anage of 13.0+/-4.7 Gyr for this star. The unambiguous actinide-boost signatureof this extremely metal-poor star, combined with additional r-process-enhancedand actinide-boost stars identified by the RPA, will provide strong constraintson the nature and origin of the r-process at early times.

The R-Process Alliance: 2MASS J09544277+5246414, the Most Actinide-Enhanced R-II Star Known