The chemical DNA of the Magellanic Clouds III. The first, extragalactic Mg-K anticorrelation: the LMC globular cluster NGC 1786

TL;DR

This paper reports the first detection of a Mg-K anticorrelation in an extragalactic globular cluster, NGC 1786 in the LMC, based on high-resolution MIKE spectra of six RGB stars. By deriving $[Mg/Fe]$ and $[K/Fe]$ with spectral synthesis and applying NLTE corrections for K, the study finds Mg-rich stars with lower K enrichment and Mg-poor stars enriched in K, consistent with a Mg-Al chain–driven self-enrichment scenario. The anticorrelation, together with Na-O and Mg-Al patterns, suggests that the same high-temperature H-burning processes operating in Milky Way clusters are at work in this external environment, pointing to a mass/metallicity threshold governing multiple populations rather than galaxy-specific factors. The results strengthen the case for AGB/super-AGB polluters as plausible drivers of extreme abundance patterns, though precise dilution and reaction-rate details remain topics of active debate, particularly given the need to reproduce both Mg depletion and K production at $T>10^8$ K. These findings support the view that self-enrichment in GCs is a universal phenomenon linked to cluster intrinsic properties rather than the formation environment.

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 found 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 extreme chemical anomaly has been detected. The observed trend nicely agrees with those observed in Galactic globular clusters hosting Mg-poor stars, such as NGC 2808, and Omega Centauri suggesting that such chemical anomaly is an ubiquitous feature of old, massive, and metal-poor stellar systems and it does not depend on the properties of the parent galaxy in which the cluster formed. Also, Na-O and Mg-Al anticorrelations were 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 depleting Mg. However, the precise and complete physics of this model remains a subject of debate.

Figures (2)

• Figure 1: Run of [K/Fe] as a function of [Mg/Fe] for the six stars of NGC 1786 here analyzed (red squares). As a comparison the same trend is plotted for the stars belonging to $\omega$ Centauri (gray and blue circles; alvarezgaray_22) and to NGC 2808 (green triangles; mucciarelli_15). The blue circles represent the stars of $\omega$ Centauri in the same metallicity range of NGC 1786. The vertical dashed line splits the Mg-poor and Mg-rich stars. The error bar on the bottom-left corner represents the typical error associated to the abundance ratios.
• Figure 2: Comparison between the spectra of NGC1786-978 (left panels) and NGC1786-2310 (right panels) around the Mg line at 5711 $\hbox{\normalfont\AA}$ (upper panels) and the K line at 7699 $\hbox{\normalfont\AA}$ (lower panels). The solid black lines represent the observed spectra, the solid red lines the best-fit synthetic spectra, and the dash-dotted blue lines the synthetic spectra with the abundances varied by $\pm 0.2$ dex with respect to the best values, respectively. In the bottom-right corners are reported the name of the stars and the corresponding [Mg/Fe] and [K/Fe] abundance ratios.