Peculiarities of the chemical enrichment of metal-poor stars in the Milky Way Galaxy

Context . The oldest stars in the Milky Way are metal-poor with Fe/H Fe/H > −3.0). Methods . The abundances were derived from spectra obtained with the HRS echelle spectrograph at the Southern African Large Telescope, using both local and non-local thermodynamic equilibrium (LTE and NLTE) approaches, with the average error between 0.10 and 0.20 dex. Results . Based on their kinematical properties, we show that HE 1523-0901 and HD 195636 are halo stars with typical high velocities. In particular, HD 121135 displays a peculiar kinematical behaviour, making it unclear whether it is a halo or an accreted star. Furthermore, HD 6268 is possibly a rare prototype of very metal-poor thick disk stars. The abundances derived for our stars are compared with theoretical stellar models and with other stars with similar metallicity values from the literature. Conclusions . HD 121135 is Al-poor and Sc-poor, compared to stars observed in the same metallicity range (−1.62 > Fe/H > −1.12). The most metal-poor stars in our sample, HE 1523-0901, HD 6268, and HD 195636, exhibit anomalies that are better explained by supernova models from fast-rotating stellar progenitors for elements up to the Fe group. Compared to other stars in the same metal-licity range, their common biggest anomaly is represented by the low Sc abundances. If we consider the elements beyond Zn, HE 1523-0901 can be classified as an r-II star, HD 6268 as an r-I candidate, and HD 195636 and HD 121135 exhibiting a borderline r -process enrichment between limited-r and r-I star. Significant relative differences are observed between the r-process signatures in these stars.