Even-to-odd Barium Isotope Ratios in Thick Disk and Halo Stars as a Constraint to r- and s-process Nucleosynthesis

We present the even-to-odd Ba isotope ratios in 16 thick disk and 3 halo stars as determined from hyperfine structure (HFS) seen in the Ba II resonance line λ 4554. We find in our stars a higher fraction of the odd Ba isotopes compared with the solar one 18% (Cameron 1982): 35% in the halo stars and between 25% and 53% in the thick disk stars. There is a hint of increasing this value with the Eu/Fe abundance ratio growth. Based on the r-process even-to-odd Ba isotope ratio 54 : 46 (Arlandini, Käppeler, Wisshak, et al. 1999) we deduce the ratio of the s/r-process contribution to barium in the thick disk stars as 30 : 70 (±24%).