Heavy elements in halo stars: the r/s-process controversy.

It has been suggested by Truran (1981) that rapid neutron captures (the r-process) dominate the production of heavy elements in very metal-poor stars. Several spectroscopic works analyzing that hypothesis are reviewed and rediscussed and it is shown that there is, in fact, no secure observational evidence in support of Truran's suggestion. A method to determine the odd-to-even isotopic ratio of barium, and thus to estimate the relative contributions of the r and s-processes, is presented. It takes advantage of the hyperfine structure affecting the spectral lines of the odd isotopes to distinguish them from the even isotopes. This method, applied to the classical metal-poor subgiant HD 140283, shows that the barium isotopic ratio in that star is in agreement with a pure s-process production, and excludes any significant enhancement of the r-process contribution, in disagreement with previous works based on elemental abundances.