Barium even-to-odd isotope abundance ratios in thick disk and thin disk stars

We present the Ba even-to-odd isotope abundance ratios in 25 cool dwarf stars with the metallicity [Fe/H] ranging between 0.25 and –1.35. Our method takes advantage of the hyperfine structure (HFS) affecting the $\ion{Ba}{ii}$ resonance line of the odd isotopes. The fractional abundance of the odd isotopes of Ba is derived from a requirement that Ba abundances from the resonance line $\lambda\,4554$ and subordinate lines $\lambda\,5853$ and $\lambda\,6496$ must be equal. The results are based on NLTE line formation and analysis of high resolution ($R \sim 60\,000$) high signal-to-noise (${\it S/N} \ge 200$) observed spectra. We find that the fraction of the odd isotopes of Ba grows toward the lower Ba abundance (or metallicity) and the mean value in the thick disk stars equals $33 \pm 4\%$. This indicates the higher contribution of the r-process to barium in the thick disk stars compared to the solar system matter. The obtained fraction increases with the [Eu/Ba] abundance ratio growth in agreement with expectations. A significant fraction of the even isotopes of Ba found in old Galactic stars (the thick disk stars), ~67%, is in contrast to the prediction of the “classical” model of the s-process and favors the value predicted by the “stellar” models of Arlandini et al. (1999) and Travaglio et al. (1999)