Europium, we have a problem

Modelling r-process enrichment across Local Group galaxies

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 Institut für Kernphysik, Technische Universität Darmstadt, Schlossgartenstr. 2, Darmstadt 64289, Germany
⁴ INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy

^★ Corresponding author: marco.palla@inaf.it

Received: 14 March 2025
Accepted: 28 May 2025

Abstract

Context. Europium (Eu) serves as a crucial tracer in studying the origin of rapid neutron-capture process (r-process) elements. For this reason, extensive efforts have been made in the last decade to model the chemical evolution of this element in the Galaxy. However, far less attention has been reserved thus far for Eu in different galaxies of the Local Group.

Aims. By employing detailed and well-tested chemical evolution models, we investigate Eu enrichment across Local Group dwarf spheroidal galaxies, allowing for a direct comparison between model predictions for dwarf galaxies and the Milky Way.

Methods. Building upon an r-process enrichment framework that successfully reproduces the observed Eu abundance patterns, as well as the supernova and compact binary merger rates in the Milky Way, we built chemical evolution models for the Sagittarius, Fornax, and Sculptor dwarf spheroidal galaxies. We used these models to test the enrichment scenario against the abundance patterns observed in these galaxies.

Results. Models reproducing the Galactic Eu patterns significantly underestimate the [Eu/Fe] ratios observed in Local Group dwarfs. To address this ‘missing Eu’ problem, we estimated the Eu production rate needed to match the observations and explored the potential contributions either from prompt (core-collapse supernova like) or delayed (compact binary mergers) sources, assessing their compatibility with the Milky Way observables.

Conclusions. The same r-process enrichment frameworks cannot simultaneously reproduce the Eu patterns both in the Milky Way and in dwarf galaxies. However, a scenario where additional Eu is provided by an increased production from delayed sources at low metallicity can theoretically reconcile the trends observed in the Milky Way and in Local Group dwarfs. This is because of the small discrepancies (≲0.1 dex) between model predictions and observations found in this case. Further studies targeting and modelling of neutron-capture elements in Local Group galaxies are needed to fill the gaps in our current understanding of this problem.