MgAl burning chain in M 54: The globular cluster-like properties of a nuclear star cluster

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 INAF, Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Rome, Italy

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

Received: 24 December 2025
Accepted: 28 January 2026

Abstract

In this study, we present the chemical abundances of Fe, Mg, Al, Si, and K for a sample of 233 likely member stars of M 54. All the stars were observed with the FLAMES high-resolution multi-object spectrograph mounted at the VLT. Our analysis confirmed the presence of a large metallicity range in M 54, with the majority of the stars having −1.8<[Fe/H]<−1.0 dex and a few stars having [Fe/H]>−1.0 dex. The mean value of the total sample is [Fe/H]=−1.40 (σ=0.22 dex). A Markov chain Monte Carlo analysis revealed that the observed spread in [Fe/H] is compatible with a non-null intrinsic iron dispersion. We also found that the metallicity distribution function and the broadening of the red giant branch of M 54 are not compatible with a single age, but instead suggest a wide age range from ∼ 13 Gyr to ∼ 1−2 Gyr or a smaller age range if a significant He enhancement (Y ∼ 0.35/0.40) is present in the most metal-rich stars. We identified among the stars in M 54 an entire pattern of anticorrelations linked to the MgAl burning cycle. In particular, the metal-rich component displays a higher level of H-burning, with the presence of more extended anticorrelations than the metal-poor component. No Mg-poor ([Mg/Fe]<0.0 dex) stars are identified in M 54. The evidence collected so far cannot be explained by either a globular cluster-like scenario or galactic chemical evolution. The chemical properties of M 54 can be explained within a scenario in which this system formed through the merging of two globular clusters: a metal-poor one with standard characteristics and a more metal-rich one with more pronounced chemical anomalies, possibly younger than the first one. M 54 is confirmed to be a key stellar system for explaining the chemical evolution of a nuclear star cluster.