The chemical DNA of the Magellanic Clouds IV. Unveiling extreme element production: the Eu abundance in the Small Magellanic Cloud

TL;DR

This paper presents the first extensive measurement of europium (Eu) abundances in the Small Magellanic Cloud, revealing unique nucleosynthesis patterns and differences from the Milky Way, especially in r-process element production.

Contribution

It provides a comprehensive dataset of Eu abundances across the SMC's metallicity range and compares these with Mg and Ba to understand nucleosynthesis and chemical evolution.

Findings

SMC shows enhanced [Eu/Fe] at all metallicities, similar to the Milky Way.

[Eu/Mg] remains high (~+0.5 dex) across metallicities, unlike in the Milky Way.

[Ba/Eu] increases with metallicity at lower [Fe/H] than in the Milky Way.

Abstract

In this study we investigate the chemical enrichment of the rapid neutron-capture process in the Small Magellanic Cloud (SMC). We measure [Eu/Fe] abundance ratios in 209 giant stars that are confirmed members of the SMC, providing the first extensive dataset of Eu abundances in this galaxy across its full metallicity range, spanning more than 1.5 dex. We compare Eu abundances with those of Mg and Ba to evaluate the efficiency of the r-process relative to $\alpha$-capture and s-process nucleosynthesis. The SMC shows enhanced [Eu/Fe] values at all metallicities (comparable with the values measured in the Milky Way), with a clear decline as [Fe/H] increases (from $\sim$ -1.75 dex to $\sim$ -0.5 dex), consistent with the onset of Type Ia supernovae. In contrast, [Eu/Mg] is enhanced by about +0.5 dex at all [Fe/H], significantly above the values observed in Milky Way stars, where [Eu/Mg] remains close to solar, reflecting comparable production of r-process and $\alpha$-capture elements. Moreover, [Ba/Eu] increases with metallicity, beginning at [Fe/H] $\approx$ -1.5 dex, namely at a lower metallicity with respect to the Milky Way, where [Ba/Eu] starts to increase around [Fe/H] $\approx$ -1 dex. Our findings suggest the SMC has a higher production of Eu (with respect to the $\alpha$-elements) than the Milky Way but in line with what observed in other dwarf systems within the Local Group. We confirm that galaxies with star formation efficiencies lower than the Milky Way have high [Eu/$\alpha$], probably indicating a stronger efficiency of the delayed sources of r-process at low metallicities.