High-resolution spectroscopy of metal-poor stars in the Small Magellanic Cloud

Metal-poor stars are the oldest objects populating the local universe and they offer a unique perspective to explore early star formation and stellar nucleosynthesis in different galaxies. However, a few extremely metal-poor stars are known in external galaxies of the local group, while for the closest Milky Way (MW) satellites, the large and small Magellanic clouds, our knowledge of the early stages of chemical enrichment is still incomplete. In this work, we study the chemical composition of seven metal-poor ([Fe/H]~-2.0 dex) red giant branch stars of the small Magellanic cloud (SMC) using wide coverage, high resolution (R∼40,000) and high SNR (∼60) UVES spectra. This is the first analysis of SMC metal-poor stars based on high-quality spectra and it provides for the first time a glimpse into the early stages of the chemical evolution of this galaxy. These metal-poor stars, likely formed in the first Gyr of the SMC life, exhibit some relevant differences with respect to the MW stars of similar [Fe/H]. α-elements are enhanced at a lower level than metal-poor MW stars, confirming a slower star formation rate in the SMC. Subsolar sodium and iron-peak elements abundances suggest a lower contribution from massive stars than in the MW. Also, very massive stars exploding as hypernovae provide a small contribution to the chemical enrichment, given the low zinc abundances of our sample. Finally, the SMC is characterised by a large star-to-star scatter in europium abundances, similarly to the MW, potentially caused by a stochastic r-process. This suggests that the contribution of neutron star mergers in the SMC is comparable to that in the MW. The derived abundances of this sample point out that the SMC in its early stages has been characterized by a chemical enrichment with a contribution by massive stars lower than the MW, as expected from a galaxy with a low star formation rate.