The chemical DNA of the Magellanic Clouds

V. The r-process dominates neutron-capture elements production in the oldest SMC stars^★

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, Via Gobetti 93/3, 40129 Bologna, Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Física y Astronomía – Instituto de Astrofísica, Autopista Concepción-Talcahuano 7100, Talcahuano, Chile
⁵ INAF-OATs, Via G.B. Tiepolo 11, Trieste 34143, Italy

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

Received: 29 October 2025
Accepted: 12 December 2025

Abstract

We present the chemical abundances of Fe, α-, 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 are characterised by [Fe/H] between −2.3 and −1.4 dex, with ten of them with [Fe/H] < −1.8 dex. According to theoretical age-metallicity relations for this galaxy, these stars were formed in the first Gyr of life of the SMC and represent the oldest SMC stars known so far. The [α/Fe] abundance ratios are enhanced but at a lower level than MW metal-poor stars, as expected according to the slow star formation rate (SFR) 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 labelled 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, and Nd), with the stars richest in Eu also being rich in these s-elements. Also, all the stars exhibit sub-solar [s/Eu] abundance ratios. At the metallicities of these stars, the production of neutron-capture elements is driven by the r-process because the low-mass AGB stars have not yet evolved and left their s-process signature in the interstellar medium (ISM). In this work, we also present a set of stochastic chemical evolution models tailored for the SMC to validate this scenario.