Exploring multiple stellar populations in globular clusters with <i>Euclid</i>: A theoretical overview and insights from NGC 6397

We investigate the behavior of multiple stellar populations in globular clusters (GCs) using photometric diagrams constructed with Euclid photometry. By employing synthetic spectra and isochrones that incorporate the chemical differences between first-population (1P) stars (resembling field stars) and second-population (2P) stars (enriched in helium and nitrogen, but depleted in carbon and oxygen) we have identified, from a theoretical perspective, the color-magnitude diagrams and the chromosome maps most effective at distinguishing these populations within GCs. Euclid photometry has proven to be a powerful tool for identifying multiple populations among M-dwarfs, as 1P and 2P stars form distinct sequences in well-chosen photometric diagrams, driven by differences in the strength of oxygen-based molecular features, such as water vapor. To validate our theoretical findings, we analyzed Euclid photometry and astrometry of the GC NGC 6397, complemented by photometric and astrometric data from the Hubble Space Telescopeand James Webb Space Telescope, enabling a comprehensive study of its stellar populations across a wide field of view (FoV). We find that the 1P constitutes ~30% of the M-dwarfs in NGC 6397, with the fraction of 1P stars remaining consistent across different stellar masses and throughout the entire FoV. Next, 2P stars exhibit an [O/Fe] depletion of about 0.3 dex relative to 1P stars and both populations display isotropic proper motions. This study represents the first comprehensive analysis of multiple populations among M-dwarfs across a wide FoV, demonstrating that Euclid photometry is a powerful instrument for investigating multiple populations in GCs.