Detailed abundance determination of metal-poor stars with X-Shooter – I. Unusual chemistry in halo stars

ABSTRACT We present a detailed chemical analysis study of 16 candidate metal-poor stars, previously identified with 2dF+AAOmega, using X-Shooter spectra and the Korg 1D local thermodynamic equilibrium spectral synthesis code. We confirm the earlier metallicity estimates and reveal six extremely metal-poor ($\left[\rm {Fe/H}\right]< -3$) stars in the current sample. Two of these stars, including the most metal poor at $\left[\rm {Fe/H}\right]= -3.89 \pm 0.07$, are kinematically associated with the Gaia–Sausage–Enceladus (GSE) accretion event, increasing the number of known GSE stars with $\left[\rm {Fe/H}\right]< -3.5$ to eight. From the X-Shooter spectra we determine abundances for 16 elements, with the element-to-iron abundance ratios generally consistent with high-resolution studies of Galactic halo stars. Within the sample, we identify three peculiar stars: the first is a GSE nitrogen enhanced metal-poor ($\left[\rm {N/Fe}\right] = 1.60 \pm 0.10$ and $\left[\rm {C/Fe}\right] = 0.23 \pm 0.08$) star with unusually high Na ($\left[\rm {Na/Fe}\right] = 2.26 \pm 0.07$) and Li ($\rm {A(Li)_{3DNLTE}} = 1.90 \pm 0.08$) abundances, but lacking enhancements in $\left[\rm {Al/Fe}\right]$ or $\left[\rm {Mg/Fe}\right]$. The second is a halo r-II star significantly enhanced in Sr ($\left[\rm {Sr/Ba}\right] = 0.39 \pm 0.08$), suggesting mixture of r-process and s-process enrichment, uncommon for r-II stars. Whilst the third is a halo star very depleted in N ($\left[\rm {N/Fe}\right] < -1.11$), with low C ($\left[\rm {C/Fe}\right] = -0.33 \pm 0.08$) and otherwise ‘normal’ $\left[\rm {X/Fe}\right]$ abundances, suggesting enrichment with Type II supernova that proceeds enrichment from massive asymptotic giant branch stars. This study reveals the substantial degree of chemical diversity in the stellar populations which assembled the early Milky Way.