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Context. Thanks to the heroic observational campaigns carried out in recentyears we now have large samples of metal-poor stars for which measurements ofdetailed abundances exist. [...] These data hold important clues on the natureof the contribution of the first stellar generations to the enrichment of ourGalaxy. Aims. We aim to explain the scatter in Sr, Ba, Y, and Eu abundanceratio diagrams unveiled by the metal-poor halo stars. Methods. We computedinhomogeneous chemical evolution models for the Galactic halo assumingdifferent scenarios for the r-process site: the electron-capture supernovae(EC) and the magnetorotationally driven (MRD) supernovae scenario. We alsoconsidered models with and without the contribution of fast-rotating massivestars (spinstars) to an early enrichment by the s-process. A detailedcomparison with the now large sample of stars with measured abundances of Sr,Ba, Y, Eu, and Fe is provided (both in terms of scatter plots and numberdistributions for several abundance ratios). Results. The scatter observed inthese abundance ratios of the very metal-poor stars (with [Fe/H] < -2.5) can beexplained by combining the s-process production in spinstars, and the r-processcontribution coming from massive stars. For the r-process we have developedmodels for both the EC and the MRD scenario that match the observations.Conclusions. With the present observational and theoretical constraints wecannot distinguish between the EC and the MRD scenario in the Galactic halo.Independently of the r-process scenarios adopted, the production of elements byan s-process in spinstars is needed to reproduce the spread in abundances ofthe light neutron capture elements (Sr and Y) over heavy neutron captureelements (Ba and Eu). We provide a way to test our suggestions by means of thedistribution of the Ba isotopic ratios in a [Ba/Fe] or [Sr/Ba] vs. [Fe/H]diagram.

Explaining the Ba, Y, Sr, and Eu abundance scatter in metal-poor halo stars: constraints to the r-process