2025ApJ...984L..43L

We report the discovery of an actinide-boost, very metal-poor ($\left[{\rm{Fe}}/{\rm{H}}\right]=-2.38$), r-process-enhanced ($\left[{\rm{Eu}}/{\rm{Fe}}\right]=0.80$) star, LAMOST J0804+5740, within the Gaia-Sausage-Enceladus (GSE). Based on the high-resolution (R ∼ 36,000 and 60,000) and high signal-to-noise ratio spectra obtained with the High Dispersion Spectrograph on the Subaru Telescope, the abundances of 48 species are determined. Its $\mathrm{log}\epsilon ({\rm{Th}}/{\rm{Eu}})=-0.22$ establishes it as the first confirmed actinide-boost star within the GSE. Comparative analysis of its abundance pattern with theoretical r-process models reveals that the magnetorotationally driven jet supernova r-process model with $\hat{L}v$ = 0.2 provides the best fit and successfully reproduces the actinide-boost signature. Kinematic analysis of actinide-boost stars reveals that approximately two-thirds of them are classified as ex situ stars, suggesting that actinide-boost stars are more likely to originate from accreted dwarf galaxies. As the first actinide-boost star identified within the GSE, J0804+5740 will provide valuable insights into r-process nucleosynthesis in accreted dwarf galaxies like the GSE, especially on the production of the heaviest elements.