We report the discovery of a new actinide-boost star , 2MASS J09544277 + 5246414 , originally identified as a very bright ( V = 10.1 ) , extremely metal-poor ( [ \mathrm { Fe / H } ] = -2.99 ) K giant in the LAMOST survey , and found to be highly r -process-enhanced ( r -II ; [ Eu/Fe ] = +1.28 ] ) , during the snapshot phase of the R -Process Alliance ( RPA ) .
Based on a high S/N , high-resolution spectrum obtained with the Harlan J. Smith 2.7-m telescope , this star is the first confirmed actinide-boost star found by RPA efforts .
With an enhancement of \textrm { [ Th / Eu ] } = +0.37 , 2MASS J09544277 + 5246414 is also the most actinide-enhanced r -II star yet discovered , and only the sixth metal-poor star with a measured uranium abundance ( \textrm { [ U / Fe ] } = +1.40 ) .
Using the Th/U chronometer , we estimate an age of 13.0 \pm 4.7 Gyr for this star .
The unambiguous actinide-boost signature of this extremely metal-poor star , combined with additional r -process-enhanced and actinide-boost stars identified by the RPA , will provide strong constraints on the nature and origin of the r -process at early times .