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arXiv · 2505.07281

An Actinide-boost Star Discovered in the Gaia-Sausage-Enceladus

Yangming Lin·Haining Li·Ruizheng Jiang·Wako Aoki·Satoshi Honda·Zhenyu He·Ruizhi Zhang·Zhuohan Li·Gang Zhao
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Abstract

We report the discovery of an actinide-boost, very metal-poor ($\left[\mathrm{Fe/H} \right]=-2.38$), $r$-process-enhanced ($\left[\mathrm{Eu/Fe} \right]=0.80$) star, LAMOST J0804+5740, within the Gaia-Sausage-Enceladus (GSE). Based on the high-resolution ($R\sim36,000\; and \;60,000$) 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 $\logε\rm(\mathrm{Th}/\mathrm{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 $\textit{ex-situ}$ 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.

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Yangming Lin, Haining Li, Ruizheng Jiang, Wako Aoki, Satoshi Honda, Zhenyu He, Ruizhi Zhang, Zhuohan Li, Gang Zhao. 2025-05-12. An Actinide-boost Star Discovered in the Gaia-Sausage-Enceladus. https://doi.org/10.3847/2041-8213%2Fadc8a3

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