  Context. Theoretical results showed the possibility that neutron capture
elements were produced in the early Universe by two different sources: a
frequent s-process source hosted by rotating massive stars, and a rare
r-process source hosted most likely by neutron star mergers. The two sources
produce barium with different isotopic compositions. Aims. We aim to
investigate the lines of barium in two halo stars, HD 6268 and HD 4306. The
spectra present an exquisite quality, both in terms of resolution (R > 100'000)
and signal-to-noise (400). Due to hyperfine splitting (hfs) effects, barium
lines are expected to show slightly different profiles depending on the barium
isotopic fraction. Methods. We applied a standard local thermodynamic
equilibrium synthesis of the barium lines. We compared the synthetic results
assuming an s-process isotopic pattern or an r-process isotopic pattern for the
two barium lines for each star that exhibited hfs. We also applied a
methodology, less dependent on the accuracy of the theoretical Ba hfs
structure, that transforms the lines of HD 4306 into those we would observe if
its atmospheric parameter values (i.e. Teff, log g, micro- and
macro-turbulence, Vsin i, and Ba abundance) were the same as those of HD 6268.
Results. With both methods, our results show that the barium lines with hfs
effects of HD 4306 are in agreement with an s-process composition and the lines
in HD 6268 have a different profile, which is most likely linked to the
presence of an r-process isotopic pattern. Conclusions. Two lines of barium of
HD 6268 and HD 4306 seem to confirm the theoretical expectation that both
r-process events and also s-process contribution by rotating massive stars have
polluted the ancient halo of our Galaxy.
