The thorium and six second-peak r-process element (56<Z<72) abundances are determined for the alpha-poor star LAMOST J1124+4535 based on a high-resolution spectrum obtained with the High Dispersion Spectrograph (HDS) on the Subaru telescope. The age of J1124+4535 is 11.3±4.4 Gyr using thorium and other r-process element abundances. J1124+4535 is confirmed to be a Galactic halo metal-poor ([Fe/H] = -1.27±0.1) star with extreme r-process element over-abundance ([Eu/Fe] = 1.13±0.08) and alpha element deficiency ([Mg/Fe] = -0.31±0.09) by the LAMOST-Subaru project. Along with the sub-solar alpha to iron ratios (e.g. [Mg/Fe], [Si/Fe], [Ca/Fe]), the relatively low abundances of Na, Cr, Ni and Zn in J1124+4535 show significant departure from the general trends of the Galactic halo but are in good agreement with those of dwarf galaxies. The chemical abundances and kinematics of J1124+4535 suggest it was formed in the late stage of star formation in a dwarf galaxy which has been disrupted by the Milky Way (MW). The star formation of its progenitor dwarf galaxy lasted more than 2 Gyr and has been affected by a rare r-process event before the occurrence of accretion event.
@article{arxiv.2404.11069,
title = {Detection of the actinide Th in an r-process-enhanced star with accretion origin},
author = {Xing Qianfan and Zhao Gang and Aoki Wako and Li Haining and Zhao Jingkun and Matsuno Tadafumi and Suda Takuma},
journal= {arXiv preprint arXiv:2404.11069},
year = {2024}
}