Four-hundred Very Metal-poor Stars Studied with LAMOST and Subaru. IV. Heavy Element Enhanced Stars

Abstract

We perform a homogeneous analysis of 82 heavy-element-enhanced metal-poor stars from the\\ LAMOST/Subaru sample, including 64 $r$-process-enhanced stars, 2 limited-$r$ stars, and 16 stars exhibiting significant or moderate carbon enhancement (comprising 9 $s$-process-enhanced, 2 $r$-process-enhanced, 2 $r$/$s$, and 3 $r$+$s$ stars). Systematic differences in $\mathrm{[X/Eu]}$ between $r$-I and $r$-II stars, together with their distinct dynamical properties, suggest that $r$-II stars likely formed in low-mass dwarf galaxies where contamination from additional nucleosynthetic channels was minimal. We have also discovered a number of peculiar objects. We identify an extremely metal-poor $r$-II star, J1158+0734 ($\mathrm{[Fe/H]}$ = -2.93), whose enhanced Zn abundance ($\mathrm{[Zn/Fe]}$ = +0.67) is best explained by yields from a high-energy, massive core-collapse supernova. The surface abundances of $s$-process-enhanced stars exhibit pronounced dispersion, consistent with enrichment from AGB progenitors with diverse properties. Notably, one star, J2256+0215, reveals compelling evidence for additional evolutionary mixing, as indicated by its unusually low carbon abundance ($\mathrm{[C/Fe]}$ = +0.08) and extremely low $^{12}$C/$^{13}$C ratio of 2.33. Additionally, we identify 3 $r$+$s$ stars whose abundance patterns reveal combined contributions from both the $r$- and $s$-processes. We further propose a modified diagnostic scheme for the identification of CEMP-$r$+$s$ stars, defined as: -0.2 $\leqslant$ $\mathrm{[La/Nd]}$ $\leqslant$ 0.2 and -0.5 $\leqslant$ $\mathrm{[Eu/Nd]}$ $\leqslant$ 0.0.