The chemical DNA of the Magellanic Clouds

III. The first extragalactic Mg–K anticorrelation: The LMC globular cluster NGC 1786

¹ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
³ INAF, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, 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, 34143 Trieste, Italy

^★ Corresponding author: deimer.alvarez@inaf.it

Received: 4 August 2025
Accepted: 2 October 2025

Abstract

In this work we derived [K/Fe] and [Mg/Fe] abundance ratios for six stars of the old globular cluster NGC 1786 in the Large Magellanic Cloud. We employed high-resolution spectra acquired with the MIKE spectrograph mounted at the Magellan/Clay telescope. We find a clear Mg–K anticorrelation among the analyzed stars. In particular, the Mg-poor stars ([Mg/Fe] < 0.0 dex) are enriched by ~0.25 dex in [K/Fe] compared to the Mg-rich stars ([Mg/Fe] > 0.0 dex). This finding makes NGC 1786 the first globular cluster residing in an external galaxy in which such an extreme chemical anomaly has been detected. The observed trend is in line with those observed in Galactic globular clusters hosting Mg-poor stars, such as NGC 2808 and ω Centauri, suggesting that such a chemical anomaly is an ubiquitous feature of old, massive, and metal-poor stellar systems and does not depend on the properties of the parent galaxy in which the cluster formed. Na–O and Mg–Al anticorrelations were also detected among the stars of NGC 1786. The newly discovered Mg–K anticorrelation reinforces the idea that stars capable of activating the complete MgAl cycle are responsible for the observed chemical anomalies in these clusters. In this context, asymptotic giant branch stars seem to be a valuable model since they are able to produce K while becoming depleted in Mg. However, the precise and complete physics of this model remains a subject of debate.