A JWST view of the globular cluster M22 and its multiple stellar populations

The discovery that globular clusters (GCs) host multiple stellar populations (MPs) with distinct chemical patterns has transformed our view of these systems. Once thought to be simple, GCs are now recognized as chemically complex, though the origin of this complexity remains uncertain. Two main scenarios attempt to explain MPs. Multiplegeneration scenarios: MPs arise from successive star-formation episodes. Secondpopulation (2P) stars form from material processed by first-population (1P) stars, with possible polluters including massive binaries, AGB stars, or rapidly rotating massive stars. These models face the mass-budget problem: only a small fraction of 1P mass is available for 2P stars, implying clusterswere initially much more massive and lost a large fraction of their original 1P stars. Single-generation scenarios with stellar accretion: all stars form in a single burst, but some accrete processed material from massive stars of the same generation. This can reproduce 2P chemical patterns without additional star formation or much larger initial cluster mass. The two scenarios predict different trends with stellar mass. Multiple-generation models imply stars of all masses share similar compositions, while accretion-based models predict that low-mass stars accrete less, so abundance differences diminish among M-dwarfs. To investigate the multiplepopulation phenomenon, I studied the globular cluster M22 using deep images from NIRCam on board JWST in the F115W, F200W, F277W, and F444W bands. I reduced the F277W and F444W images with state-of-the-art programs developed by Jay Anderson, corrected the photometry for differential reddening, and selected cluster members using proper motions from multi-epoch JWST and HST observations. The resulting CMDs show a narrow, well-defined MS above the knee, consistent with moderate helium variation, while belowthe MS knee the sequence broadens, indicating multiple populations with different oxygen abundances. Using a pseudo two-color diagram (chromosome map), I identified first- and second-population stars among M-dwarfs with masses below 0.4⊙. This is the first detection of distinct stellar populations among very low-mass stars in M22. I find that the fraction of first-population stars is 25 ± 3%, consistent within one sigma with values reported for RGB stars above 0.8⊙. This supports predictions of multiple-generation scenarios and challenges the expectations of accretion-based models.