Ruthenium and hafnium abundances in giant and dwarf barium stars

Aims. We present abundances for Ru and Hf, compare them to abundances of other heavy elements, and discuss the problems found in determining Ru and Hf abundances with laboratory g f -values in the spectra of barium stars. Methods. We determined Ru and Hf abundances in a sample of giant and dwarf barium stars, by the spectral synthesis of two Ru  (λ4080.574 and λ4757.856) and two Hf  (λ4080.437 and λ4093.155) transitions. The stellar spectra were observed with FEROS/ESO, and the stellar atmospheric parameters lie in the range 4300 < Teff /K < 6500, −1.2 < [Fe/H] ≤ 0 and 1.4 ≤ log g < 4.6. Results. The Hf  λ4080 and the Ru  λ4758 observed transitions result in a unreasonably high solar abundance, given certain known uncertainties, when fitted with laboratory g f -values. For these two transitions we determined empirical g f -values by fitting the observed line profiles of the spectra of the Sun and Arcturus. For the sample stars, this procedure resulted in a good agreement of Ru and Hf abundances given by the two available lines. The resulting Ru and Hf abundances were compared to those of Y, Nd, Sm and Eu. In the solar system Ru, Sm and Eu are dominated by the r-process and Hf, Nd and Y by the s-process, and all of these elements are enhanced in barium stars since they lie inside the s-process path. Ru abundances show large scatter when compared to other heavy elements, whereas Hf abundances show less scatter and closely follow the abundances of Sm and Nd, in good agreement with theoretical expectations. We also suggest a possible, unexpected, correlation of Ru and Sm abundances. The observed behaviour in abundances is probably due to variations in the 13C pocket efficiency in AGB stars, and, though masked by high uncertainties, hint at a more complex scenario than proposed by theory. Conclusions.