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 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 60000 ) high signal-to-noise ( S/N \geq 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 ) , \sim 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 ) .