Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

We present the first chemical evolution model for Enceladus, alias the Gaia Sausage, to investigate the star formation history of one of the most massive satellites accreted by the Milky Way during a major merger event. Our best chemical evolution model for Enceladus nicely fits the observed stellar [α/Fe]–[Fe/H] chemical abundance trends, and reproduces the observed stellar metallicity distribution function, by assuming low star formation efficiency, fast infall time-scale, and mild outflow intensity. We predict a median age for Enceladus stars |$12.33^{+0.92}_{-1.36}$| Gyr, and – at the time of the merger with our Galaxy (≈10 Gyr ago from Helmi et al.) – we predict for Enceladus a total stellar mass M_⋆ ≈ 5 × 10⁹ M_|$\odot$|. By looking at the predictions of our best model, we discuss that merger events between the Galaxy and systems like Enceladus may have inhibited the gas accretion on to the Galaxy disc at high redshifts, heating up the gas in the halo. This scenario could explain the extended period of quenching in the star formation activity of our Galaxy about 10 Gyr ago, which is predicted by Milky Way chemical evolution models, in order to reproduce the observed bimodality in [α/Fe]–[Fe/H] between thick- and thin-disc stars.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Context. The dominant astrophysical production site of the r-process elements has not yet been unambiguously identified. The suggested main r-process sites are core-collapse supernovae and merging neutron stars.Aims. We explore the problem of the production site of Eu. We also use the information present in the observed spread in the Eu abundances in the early Galaxy, and not only its average trend. Moreover, we extend our investigations to other heavy elements (Ba, Sr, Rb, Zr) to provide additional constraints on our results. Methods. We adopt a stochastic chemical evolution model that takes inhomogeneous mixing into account. The adopted yields of Eu from merging neutron stars and from core-collapse supernovae are those that are able to explain the average [Eu/Fe]–[Fe/H] trend observed for solar neighbourhood stars, the solar abundance of Eu, and the present-day abundance gradient of Eu along the Galactic disc in the framework of a well-tested homogeneous model for the chemical evolution of the Milky Way. Rb, Sr, Zr, and Ba are produced by both the s- and r-processes. The r-process yields were obtained by scaling the Eu yields described above according to the abundance ratios observed in r-process rich stars. The s-process contribution by spinstars is the same as in our previous papers.Results. Neutron star binaries that merge in less than 10 Myr or neutron star mergers combined with a source of r-process generated by massive stars can explain the spread of [Eu/Fe] in the Galactic halo. The combination of r-process production by neutron star mergers and s-process production by spinstars is able to reproduce the available observational data for Sr, Zr, and Ba. We also show the first predictions for Rb in the Galactic halo. Conclusions. We confirm previous results that either neutron star mergers on a very short timescale or both neutron star mergers and at least a fraction of Type II supernovae have contributed to the synthesis of Eu in the Galaxy. The r-process production of Sr, Zr, and Ba by neutron star mergers – complemented by an s-process production by spinstars – provide results that are compatible with our previous findings based on other r-process sites. We critically discuss the weak and strong points of both neutron star merging and supernova scenarios for producing Eu and eventually suggest that the best solution is probably a mixed one in which both sources produce Eu. In fact, this scenario reproduces the scatter observed in all the studied elements better.

Context. Very old halo stars have previously been found to show at least four different abundance anomalies, which models of fast-rotating massive stars (spinstars) can successfully account for: rise of N/O and C/O, low 12C/13C, 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.