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Context. Prior studies have found that there are two distinct halo populations with a clear separation in [α/Fe], known as the high-α and low-α populations. Certain other elements, such as [C/Fe], [O/Fe], and [Na/Fe], as well as [Cu/Fe], also show a clear distinction between the high- and low-α halo populations. However, the abundance ratios, such as [Li/Fe] and [Mn/Fe], are indistinguishable between these two halo populations.
Aims. This work is aim at determining the abundances of barium and europium for a sample of halo stars, including 29 high-α and 19 low-α halo stars, and to investigate the distributions of [Ba/Fe] and [Eu/Fe]. We also present the fraction of barium isotopes with an odd mass number (f_odd) in the sample stars to estimate the relative contribution of the r- and s-processes to the abundance of Ba.
Methods. The abundance ratios of [Ba/Fe] and [Eu/Fe] were determined by the spectral synthesis method based on the high-resolution and high signal-to-noise ratio (S/N) spectra collected from the ESO/ST-ECF Science Archive of VLT/UVES and NOT/FIES spectrographs. Non-local thermodynamic equilibrium (NLTE) effects were also considered. The f_odd value was estimated by fitting the profile of the Ba II resonance line at λ4554 Å.
Results. The NLTE effect is needed to be included to fully capture the line profiles for both Ba II and Eu II, however, the NLTE corrections for the barium and europium are different. For Ba, the NLTE corrections are negative compared to the results of thermodynamic equilibrium, except the star with lowest metallicity, and they become smaller with decreasing of [Fe/H]. For Eu, the NLTE corrections are positive and independent of stellar metallicity. It is found that both the abundances of [Ba/Fe] and [Eu/Fe] in the high-and low-α halo stars are mixed together. Although the [Eu/Ba] ratios cannot be distinguished clearly between the high- and low-α halo populations, stars with [Eu/Ba] ≥ 0.7 dex can only be found in the low-α population. Similar to [Eu/Ba], the ratios of f_odd cannot be separated among the high- and low-α halo populations, although the stars with the highest f_odd (≥0.4) can only be seen in the low-α stars with [Eu/Ba] ≥ 0.7 dex.
Conclusions. Our results indicate that both the [Ba/Fe] and [Eu/Fe] ratios are indistinguishable among the high- and low-α halo stars, while [Eu/Mg] is able to separate the two halo populations well. Combining the highest [Eu/Ba] and f_odd ratios, as well as the lower [α/Fe] in some low-α halo stars, we can infer that the r-process is dominating in these low-α halo stars and they are likely to have been accreted from dwarf galaxies as proposed by previous studies.