Even-to-Odd Barium Isotope Ratio in Selected Galactic Halo Stars

We have improved the Ba II model atom by taking into account the excitation of transitions through collisions with hydrogen atoms with the rate coefficients from the quantum-mechanical calculations of Belyaev and Yakovleva (2018). Using high-resolution spectra and Ba II line modeling when abandoning the assumption of LTE, we have determined the fraction of barium isotopes with an odd mass number (fodd) in four Galactic halo giants with well-known atmospheric parameters. We use a method based on the requirement that the abundances from the resonance (Ba II 4554 Å) and subordinate (Ba II 5853, 6496 Å) lines be equal. A accuracy of 0.04 dex in determining the barium abundance from individual lines has been achieved. In three stars (HD 2796, HD 108317, and HD 122563) fodd ≳ 0.4. This suggests that ≳80% of the barium observed in these stars was synthesized in the r-process. In HD 128279 fodd = 0.27 exceeds the fraction of odd barium isotopes in the Solar system, but only slightly. The dominance of the r-process at the formation epoch of the stars from our sample is confirmed by the presence of a europium overabundance relative barium in them, with [Eu/Ba] > 0.3. We have calculated the non-LTE barium abundance corrections for five Ba II lines and investigated their dependence on atmospheric parameters in the ranges of effective temperatures from 4500 to 6500 K, surface gravities log g from 0.5 to 4.5, and metallicities [Fe/H] from 0 to −3.