The chemical evolution of Barium and Europium in the Milky Way

Abstract

We compute the evolution of the abundances of barium and europium in the Milky Way with a chemical evolution model which already reproduces the majority of observational constraints, and we compared our results with the observed abundances from the recent UVES Large Program “First Stars” (Cayrel et al. 2004, François et al. 2005).

We confirm that barium is a neutron capture element mainly produced in the low mass AGB stars, during the thermal-pulsing phase, by the ¹³C 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, Ba should be also produced as a r-process element by massive stars in the range 10-30M_{[odot ]}. On the other hand, europium should be only a r-process element produced in the same range of masses (10-30M_{[odot ]}), 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 ranges in the r-process yields from massive stars for both elements which better reproduce the trend of the data. We discuss several possibilities to explain the observed spread. We suggest that a peculiar behaviour of the neutron capture elements could be the responsible for the spread instead of invoking a strongly inhomogeneous early Galactic halo. We finally underline that the production ratio of [Ba/Eu] may be almost constant in the massive stars.