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We used ultra-deep observations obtained with the NIRCam aboard the JamesWebb Space Telescope to explore the stellar population of NGC 6440: a typicalmassive, obscured and contaminated globular cluster formed and orbiting withinthe Galactic bulge. Leveraging the exceptional capabilities of this camera, wesampled the cluster down to ~5 magnitudes below the main-sequence turn-off inthe (mF115W , mF115W - mF200W ) colour-magnitude diagram. After carefullyaccounting for differential extinction and contamination by field interlopers,we find that the main sequence splits into two branches both above and belowthe characteristic knee. By comparing the morphology of the colour-magnitudediagram with a suitable set of isochrones, we argue that the uppermain-sequence bi-modality is likely due to the presence of a He-enrichedstellar population with a helium spread of DeltaY = 0.04. The lowermain-sequence bi-modality can be attributed to variations in the abundance ofwater (i.e., oxygen) with Delta[O/Fe] ~ -0.4. This is the first evidence ofboth helium and oxygen abundance variations in a globular cluster purely basedon JWST observations. These results open the window for future in-depthinvestigations of the multiple population phenomenon in clusters located in theGalactic bulge, which were previously unfeasible with near-UV observations, dueto prohibitive reddening and crowding conditions.

JWST uncovers helium and water abundance variations in the bulge globular cluster NGC 6440