Spectro-Photometry and Radial Distribution of Multiple Stellar Populations in Globular Clusters from Gaia XP Spectra

TL;DR

This study uses Gaia XP spectra to identify and analyze multiple stellar populations in globular clusters, revealing their spatial distributions and challenging some existing formation theories.

Contribution

It introduces new photometric bands and chromosome maps to distinguish stellar populations in cluster outskirts using Gaia data.

Findings

Successfully identified multiple populations in outer cluster regions.

Found 2P stars are more centrally concentrated in some clusters.

Challenged previous assumptions about population distributions in NGC3201.

Abstract

Understanding the formation of multiple populations in globular clusters (GCs) represents a challenge for stellar population studies. Nevertheless, the outermost cluster regions, likely to hold clues about the initial configuration of GC stars, remain underexplored. We use synthetic spectra reflecting the chemical compositions of first- and second-population (1P, 2P) stars in 47Tucanae to identify spectral regions sensitive to these populations. This led us to define new photometric bands that effectively distinguish 1P and 2P giant stars using Gaia XP spectra. Testing these filters, we constructed the pseudo two-color diagrams dubbed chromosome maps (ChMs) and, for the first time, identified 1P and 2P stars in the cluster's outermost regions and beyond its tidal radius. We constructed similar diagrams for NGC3201, NGC6121, NGC6752, and NGC6397, thus exploring GCs with different metallicities. The ChMs effectively distinguished multiple populations in the outer regions of all clusters, except for the metal-poor NGC6397. Our findings, together with literature results from more-internal regions, show that the 2P stars of 47Tucanae are more-centrally concentrated than the 1P. A similar pattern is seen for 2P stars with extreme chemical composition of NGC3201. The multiple populations of NGC6121, and NGC6752 share the same radial distributions. These radial behaviors are consistent with the GC formation scenarios where 2P stars originate in the central regions. Noticeably, results on NGC3201 are in tension with the conclusion from recent work that its 1P is more centrally concentrated than the 2P and might form with more central concentration.