The chemical DNA of the Magellanic Clouds V. R-process dominates neutron capture elements production in the oldest SMC stars
We present the chemical abundances of Fe, alpha- and neutron-capture elements in 12 metal-poor Small Magellanic Cloud (SMC) giant stars, observed with the high-resolution spectrographs UVES/VLT and MIKE/Magellan. These stars have [Fe/H] between -2.3 and -1.4 dex, 10 of them with [Fe/H]<-1.8 dex. According to theoretical age-metallicity relations for this galaxy, these stars formed in the first Gyr of life of the SMC and represent the oldest SMC stars known so far. [alpha/Fe] abundance ratios are enhanced but at a lower level than MW metal-poor stars, as expected according to the slow star formation rate of the SMC. The sample exhibits a large star-to-star scatter in all the neutron-capture elements. The two r-process elements measured in this work (Eu and Sm) have abundance ratios from solar up to +1 dex, three of them with [Eu/Fe]>+0.7 dex and labeled as r-II stars. This [r/Fe] distribution indicates that the r-process in the SMC can be extremely efficient but is still largely affected by the stochastic nature of the main sites of production and the inefficient gas mixing in the early SMC evolution. A similar scatter is observable also for the s-process elements (Y, Ba, La, Ce, Nd), with the stars richest in Eu also being rich in these s-elements. Also, all the stars exhibit subsolar [s/Eu] abundance ratios. At the metallicities of these stars, the production of neutron-capture elements is driven by r-process, because the low-mass AGB stars have not yet evolved and left their s-process signature in the interstellar medium. We also present stochastic chemical evolution models tailored for the SMC that confirm this scenario.