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

Abstract We report the discovery of an actinide-boost, very metal-poor ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="]" open="["> <mml:mrow> <mml:mi mathvariant="normal">Fe</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> </mml:mfenced> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>2.38</mml:mn> </mml:math> ), r -process-enhanced ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="]" open="["> <mml:mrow> <mml:mi mathvariant="normal">Eu</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">Fe</mml:mi> </mml:mrow> </mml:mfenced> <mml:mo>=</mml:mo> <mml:mn>0.80</mml:mn> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>ϵ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">Th</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">Eu</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.22</mml:mn> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>ˆ</mml:mo> </mml:mrow> </mml:mover> <mml:mi>v</mml:mi> </mml:math> = 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.