The s-process in the Galactic halo: the fifth signature of spinstars in the early Universe?

Context. Very old halo stars have previously been found to show at least four different abundance anomalies, which models of fastrotating massive stars (spinstars) can successfully account for: rise of N/ Oa nd C/O, low 12 C/ 13 C, and a primary-like evolution of Be and B. Here we show the impact of these same stars in the enrichment of Sr and Ba in the early Universe. Aims. We study whether the s-process production of fast-rotating massive stars can offer an explanation for the observed spread in [Sr/Ba] ratio in halo stars with metallicity [Fe/H] < −2.5. Methods. By means of a chemical inhomogeneous model, we computed the enrichment of Sr and Ba by massive stars in the Galactic halo. Our model takes, for the first time, the contribution of spinstars into account. Results. The model (combining an r-process contribution with an s-process from fast-rotating massive stars) is able to reproduce the observed scatter in the [Sr/Ba] ratio at [Fe/H] < −2.5. Toward higher metallicities, the stochasticity of the star formation fades away owing to the increasing number of exploding and enriching stars, and as a consequence the predicted scatter decreases. Conclusions. Our scenario is again based on the existence of spinstars in the early Universe. Very old halo stars have previously been found to show at least four other abundance anomalies, which rotating models of massive stars can successfully account for. Our results provide a fifth independent signature of fast-rotating massive stars: an early enrichment of the Universe in s-process elements.