  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$\pm$4.4 Gyr using thorium and other
r-process element abundances. J1124+4535 is confirmed to be a Galactic halo
metal-poor ([Fe/H] = -1.27$\pm$0.1) star with extreme r-process element
over-abundance ([Eu/Fe] = 1.13$\pm$0.08) and alpha element deficiency ([Mg/Fe]
= -0.31$\pm$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.
