Abundance of Heavy Elements in Extremely Metal-Poor Stars

International audienceIn the framework of the VLT Large program "First Stars" (165.N-0276(A)), we have measured the abundance of 13 heavy elements using high quality UVES spectra. In this paper, we report on the abundance of Sr and Ba in this sample of stars. In 1995, McWilliam et al. [4] showed that the [Sr/Fe] and [Ba/Fe] ratios exhibited a large dispersion in metal-poor stars. If these 2 elements are produced by the same nucleosynthetic process, then the variation of [Sr/Ba] as a function of metallicity should be constant . However, it is known (see Arlandini et al. 1999 [1] for example) that a significant part of Sr is built by s-process in massive stars. As this process is a secondary process, it is unlikely that this process is fully in operation at the early stages of the chemical evolution. On figure 1a, the [Sr/Ba] vs [Fe/H] are plotted together with some data found in the literature