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

TL;DR

This study analyzes 233 likely M54 members with FLAMES high-resolution spectra to characterize both the iron distribution and light-element patterns. The authors derive comprehensive abundances for Fe, Mg, Al, Si, and K using a robust SALVADOR-based pipeline and Gaia photometry to determine stellar parameters, revealing a significant intrinsic Fe dispersion together with extensive Mg–Al–Si–K processing. The results show a broad MDF with a mean $[Fe/H] = -1.41$ and an age/He-related explanation for the RGB width, along with clear Mg–Al, Mg–Si, and Mg–K anticorrelations that intensify in the metal-rich population. A GC-merger scenario within the Sagittarius NSC framework provides a coherent interpretation, reconciling GC-like multiple populations with galaxy-like metallicity evolution and highlighting M54 as a pivotal case for NSC chemical evolution.

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 few stars with [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 they 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 the 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 neither with a globular cluster-like scenario nor with a galactic chemical evolution. The chemical properties of M 54 can be explained within a scenario where this system formed through the merging of two globular clusters, the metal-poor one with standard characteristics and the more metal-rich one with more pronounced chemical anomalies, a possibly younger than the first one. M 54 is confirmed as a key stellar system for explaining the chemical evolution of a nuclear star cluster.

Figures (9)

• Figure 1: Coordinate positions of the observed targets are displayed by the purple circles. The black cross denotes the cluster center baumgardt_18. The dashed black circle displays the tidal radius harris_10.
• Figure 2: CMD of M 54. Gray points represent the targets associated to Sgr according to the proper motions of Gaia DR3, while the purple circles represent the spectroscopic target stars.
• Figure 3: Distributions of the [Fe/H] and RVs for the target stars. The main panel shows the behavior of the RV of the observed stars as a function of [Fe/H]. The generalized histograms of [Fe/H] and RV distributions are also plotted.
• Figure 4: CMD of M 54 with superimposed three different isochrones with the same age (12 Gyr) but a different [Fe/H]. The solid cyan isochrone has $\rm [Fe/H] = -1.8$ dex, the dash-dotted orange isochrone has $\rm [Fe/H] = -1.4$ dex, and the dashed red isochrone has $\rm [Fe/H] = -1.1$ dex.
• Figure 5: The four panels depict the distribution of [Mg/Fe] (top-left), [Al/Fe] (top-right), [Si/Fe] (bottom-left), and [K/Fe] (bottom-right) as function of [Fe/H] for the M 54 stars. The error bar in the corner of each panel represents the typical error associated with the measurements.