The chemical evolution of barium and europium in the Milky Way

Aims.We compute the evolution of the abundances of barium and europium in the Milky Way and we compare our results to the observed abundances from the recent UVES Large Program “First Stars”.Methods.We use a chemical evolution model that reproduces the majority of observational constraints.Results.We confirm that barium is a neutron capture element mainly produced in the low mass AGB stars during the thermal-pulsing phase by the neutron source, in a slow neutron capture process. However, in order to reproduce the [Ba/Fe] vs. [Fe/H] as well as the Ba solar abundance, we suggest that Ba is also produced as an r-process element by massive stars in the range 10–30 . On the other hand, europium should be only an r-process element produced in the same range of masses (10–30 ), at variance with previous suggestions indicating a smaller mass range for the Eu producers. As it is well known, there is a large spread in the [Ba/Fe] and [Eu/Fe] ratios at low metallicities, although smaller in the newest data. With our model we estimate for both elements (Ba and Eu) the ranges for the r-process yields from massive stars that better reproduce the trend of the data. We find that with the same yields able to explain the observed trends, the large spread in the [Ba/Fe] and [Eu/Fe] ratios cannot be explained even in the context of an inhomogeneous models for the chemical evolution of our Galaxy. We therefore derive the amount by which the yields should be modified to fully account for the observed spread. We then discuss several possibilities to explain the size of the spread. We suggest that the production ratio of [Ba/Eu] could be almost constant in massive stars.