Search for: All records

Context. The oldest stars in the Milky Way are metal-poor with [Fe/H] < −1.0, displaying peculiar elemental abundances compared to solar values. The relative variations in the chemical compositions among stars is also increasing with decreasing stellar metallicity, allowing for the pure signature of unique nucleosynthesis processes to be revealed. The study of ther-process is, for instance, one of the main goals of stellar archaeology and metal-poor stars exhibit an unexpected complexity in the stellar production of ther-process elements in the early Galaxy.

Aims. In this work, we report the atmospheric parameters, main dynamic properties, and the abundances of four metal-poor stars: HE 1523-0901, HD 6268, HD 121135, and HD 195636 (−1.5 > [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 borderliner-process enrichment between limited-r and r-I star. Significant relative differences are observed between the r-process signatures in these stars.

The study of the origin of heavy elements is one of the main goals of nuclear astrophysics. In this paper, we present new observational data for the heavy r-process elements gadolinium (Gd, Z= 64), dysprosium (Dy, Z= 66), and thorium (Th, Z= 90) in a sample of 276 Galactic disc stars (–1.0 < [Fe/H] < + 0.3). The stellar spectra have a high resolution of 42 000 and 75 000, and the signal-to-noise ratio higher than 100. The LTE abundances of Gd, Dy, and Th have been determined by comparing the observed and synthetic spectra for three Gd lines (149 stars), four Dy lines (152 stars), and the Th line at 4019.13 Å (170 stars). For about 70 per cent of the stars in our sample, Gd and Dy are measured for the first time, and Th for 95 per cent of the stars. Typical errors vary from 0.07 to 0.16 dex. This paper provides the first extended set of Th observations in the Milky Way disc. Together with europium (Eu, Z= 63) data from our previous studies, we have compared these new observations with nucleosynthesis predictions and Galactic Chemical Evolution simulations. We confirm that [Gd/Fe] and [Dy/Fe] show the same behaviour of Eu. We study with GCE simulations the evolution of [Th/Fe] in comparison with [Eu/Fe], showing that unlike Eu, either the Th production is metallicity dependent in case of a unique source of the r-process in the Galaxy, or the frequency of the Th-rich r-process source is decreasing with the increase in [Fe/H].