Neutron-capture element signatures in globular clusters

Insights from the Gaia-ESO Survey

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Florence, Italy
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via P. Gobetti 93/3, 40129 Bologna, Italy
⁴ Dipartimento di Fisica, Sezione di Astronomia, Università di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
⁵ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
⁶ INFN, Sezione di Trieste, Via A. Valerio 2, 34127 Trieste, Italy
⁷ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁹ School of Physical and Chemical Sciences – Te Kura Matū, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
¹⁰ ESO – European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via Sansone 1, 50019 Sesto Fiorentino, Italy

^★ Corresponding author: jose.schiappacasse@inaf.it

Received: 18 April 2025
Accepted: 12 May 2025

Abstract

Context. Globular clusters (GCs) are crucial to our understanding of the formation and evolution of our Galaxy. While their abundances of light and iron-peak elements have been extensively studied, research on heavier elements and their possible link to both the multiple stellar population phenomenon and the origin of GCs remains relatively limited.

Aims. We aim to analyse the chemical abundances of various neutron-capture elements using GCs as tracers of the Galactic halo. Furthermore, we explore the potential connection between these elements and the multiple stellar population phenomenon in GCs to better constrain the nature of the polluters responsible for the intracluster enrichment. Additionally, we seek to determine the origins of GCs based on their neutron-capture element abundances.

Methods. We analysed a sample of 14 GCs spanning a wide metallicity range, [Fe/H] from −0.40 to −2.32, observed as a part of the Gaia-ESO Survey and analysed using a homogeneous methodology. Here we present results for Y, Zr, Ba, La, Ce, Nd, Pr, and Eu obtained from FLAMES-UVES spectra. We compared our results with a stochastic Galactic chemical evolution model.

Results. Except for Zr, the Galactic chemical evolution model, when available, closely describes the broad trend displayed by neutron-capture elements in GCs. Moreover, in some clusters, a strong correlation between hot H-burning (Na and Al) and s-process elements suggests a shared nucleosynthetic site, for example asymptotic giant branch stars of different masses and/or fast-rotating massive stars that produced the intracluster pollution. Additionally, we identified clear differences in the [Eu/Mg] ratio between in situ (⟨[Eu/Mg]]⟩=0.14 dex) and ex situ (⟨[Eu/Mg]]⟩=0.32 dex GCs, which reveal their distinct chemical enrichment histories. Finally, on average, the Type II GCs NGC 362, NGC 1261, and NGC 1851 show a spread ratio in s-process elements between second- and first-generation stars that is roughly twice as large as that observed in Type I clusters.