A stellar census in globular clusters with MUSE: Multiple populations chemistry in NGC 2808

Abstract: Galactic globular clusters (GCs) are now known to host multiple populations displaying particular abundance variations. The different populations within a GC can be well distinguished following their position in the pseudo two-colors diagrams, also referred to as "chromosome maps". These maps are constructed using optical and near-UV photometry available from the Hubble Space Telescope (HST) UV survey of GCs. However, the chemical tagging of the various populations in the chromosome maps is hampered by the fact that HST photometry and elemental abundances are both available only for a limited number of stars. The spectra collected as part of the MUSE survey of globular clusters provide a spectroscopic counterpart to the HST photometric catalogs covering the central regions of GCs. In this paper, we use the MUSE spectra of 1115 red giant branch (RGB) stars in NGC 2808 to characterize the abundance variations seen in the multiple populations of this cluster. We use the chromosome map of NGC 2808 to divide the RGB stars into their respective populations. We then combine the spectra of all stars belonging to a given population, resulting in one high signal-to-noise ratio spectrum representative of each population. Variations in the spectral lines of O, Na, Mg, and Al are clearly detected among four of the populations. In order to quantify these variations, we measured equivalent width differences and created synthetic populations spectra that were used to determine abundance variations with respect to the primordial population of the cluster. Our results are in good agreement with the values expected from previous studies based on high-resolution spectroscopy. We do not see any significant variations in the spectral lines of Ca, K, and Ba. We also do not detect abundance variations among the stars belonging to the primordial population of NGC 2808.

A Stellar Census in Globular Clusters with MUSE: Multiple Populations Chemistry in NGC 2808

The paper by Latour et al. presents an in-depth spectroscopic analysis of the globular cluster NGC 2808, carried out using the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT). The focus of the study is to examine the chemical abundance variations among the multiple stellar populations within the cluster, which is known for its rich and complex population structure.

Globular clusters (GCs) have been traditionally considered as simple stellar populations; however, recent studies, including this one, reveal a more complicated picture with multiple populations characterized by distinct chemical signatures. NGC 2808, in particular, exhibits noticeable abundance variations among its stars, especially in the light elements such as O, Na, Mg, and Al.

Methodology

The study utilizes spectra from 1115 red giant branch (RGB) stars within NGC 2808, obtained using the integral-field spectrograph, MUSE. These spectra provide a spectroscopic counterpart to Hubble Space Telescope (HST) photometric catalogs. The authors employ chromosome maps, derived from HST photometry, to delineate different stellar populations within the GC. Each population's stars are combined to produce high signal-to-noise ratio spectra, which allows for a robust examination of abundance variations despite the low resolution of individual MUSE spectra.

Key Findings

1. Elemental Abundance Variations: The paper reports clear detection of variability in the spectral lines of O, Na, Mg, and Al among four of the identified stellar populations. The study quantifies these variations using equivalent width differences and synthetic population spectra.
2. Confirmation of Anticorrelations: The observed abundance variations align with well-known anticorrelation patterns such as Na-O and Al-Mg, which are characteristic of multiple populations in globular clusters. The findings are consistent with high-resolution spectroscopic studies previously conducted on a smaller number of stars.
3. No Significant Variations in Ca, K, and Ba: Interestingly, the study does not find significant abundance variations in calcium, potassium, or barium among the populations.
4. Primordial Population Stability: The research highlights the homogeneity within the primordial population of NGC 2808, finding no significant abundance variations, which provides insight into the initial chemical makeup of the cluster.

Implications and Future Directions

This study underscores the efficacy of MUSE for conducting detailed chemical analyses in the crowded environments of globular clusters, despite its low spectral resolution. The ability to identify and quantify abundance variations using spectroscopic data from a large number of stars equips astronomers with a powerful tool for studying the origins and evolution of stellar populations within GCs.

The findings have implications for our understanding of the formation and evolutionary processes in GCs, particularly concerning the sources of element enrichment and the mechanisms underlying the observed abundance patterns. Future research could extend the methodology used in this study to other clusters, enhancing the quest to unravel the detailed enrichment history and dynamic formation scenarios of globular clusters. This could also contribute to refining models of chemical evolution not only in GCs but also in the broader context of galaxy evolution.