Unveiling extreme element production: the r-process in the Small Magellanic Cloud

The rapid neutron capture is the nuclear reaction that produces half of the elements heavier than iron. Since the discovery of the first metal-poor r-enriched stars in the Milky Way (MW) halo, it was clear that this process occurred in the early Universe; the star-to-star scatter in [Eu/Fe] among the halo stars led to think that r-element nucleosynthesis events must be rare and stochastic. Galactic chemical evolution models suggest, as main events, the neutron star mergers (NSMs). Most of the halo stars likely formed in former satellite galaxies of the MW. It is thought that most of these accreted stars have high [Eu/α] values; the study of Eu and [Eu/α] in the Local Group is then crucial to understand the origin of accreted stars in the MW. The Small and the Large Magellanic Cloud (SMC, LMC) allow to study the origin of r-elements thanks to their long metal-poor star formation phase, but there are still open questions about the r-process in the Clouds. In this thesis, we study the chemical composition of 209 RGB stars of the SMC using medium-resolution FLAMES-GIRAFFE spectra to measure the chemical abundances of [Eu/Fe] (a r-process proxy), [Ba/Eu] (a s-process contamination proxy) and [Eu/Mg] (to compare the r-process efficiency with that of the α-elements production), to understand the efficiency of the r-process in the SMC. All stars in our sample have [Eu/Fe]>0 dex, indicating an efficient r-process; also, at low metallicities, [Eu/Fe] exhibits a large star-to-star scatter, reflecting the stochastic nature of the r-process production sites. At higher [Fe/H], the scatter reduces as [Eu/Fe] decreases for increasing metallicity. The [Ba/Eu] abundances increase for increasing [Fe/H], so the s-process has become dominant. All stars have supersolar [Eu/α], so the r-enrichment process must have occurred due to rare, stochastic events on timescales between those of CC-SNe and Type Ia SNe. The observations favor NSMs as the origin of the r-elements observed in the SMC.