χ² contour maps as a function of internal He spread, δY, and fraction of He-enriched stars, f_2P . χ² values are coded by colors. Best-fitting points (minimum χ²) are represented by red stars. Black solid lines indicate 1σ uncertainties, calculated from χ² variations. The adopted model assumes a uniform He distribution. Panels a, b, c, and d correspond to clusters Hodge 11, NGC 1841, NGC 2210, and NGC 2257, respectively.