The chemical DNA of the Magellanic Clouds

IV. Unveiling extreme element production: The Eu abundance in the Small Magellanic Cloud^★

¹ Dipartimento di Fisica e Astronomia “Augusto Righi”, Alma Mater Studiorum, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 5 September 2025
Accepted: 8 November 2025

Abstract

In this study we investigate the chemical enrichment of the rapid neutron-capture process in the Small Magellanic Cloud (SMC). We measured the [Eu/Fe] abundance ratio of 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 compared the Eu abundances with those of Mg and Ba to evaluate the efficiency of the r-process relative to α-capture and s-process nucleosynthesis. The SMC shows enhanced [Eu/Fe] values at all metallicities (comparable with the values measured in the Milky Way) and a clear decline as [Fe/H] increases (from approximately −1.75 dex to approximately −0.5 dex), which is consistent with the onset of Type Ia supernovae. In contrast, [Eu/Mg] is enhanced by about +0.5 dex at all [Fe/H] and thus significantly above the values observed in Milky Way stars, where [Eu/Mg] remains close to the solar value, reflecting comparable production of r-process and α-capture elements. Moreover, [Ba/Eu] increases with metallicity beginning at [Fe/H] ≈ −1.5 dex, namely at a lower metallicity with respect to the Milky Way, where [Ba/Eu] starts to increase around [Fe/H] ≈ −1 dex. Our findings suggest the SMC has a higher production of Eu (with respect to the α-elements) than the Milky Way, but it is still in line with what has been observed in other dwarf systems within the Local Group. We confirm that galaxies with star formation efficiencies lower than the Milky Way have a high [Eu/α], probably indicating stronger efficiency of the delayed sources of the r-process at low metallicities.