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Disentangling distinct stellar populations along the red-giant branches(RGBs) of Globular Clusters (GCs) is possible by using the pseudo two-colordiagram dubbed chromosome map (ChM). One of the most intriguing findings isthat the so-called first-generation (1G) stars, characterized by the samechemical composition of their natal cloud, exhibit extended sequences in theChM. Unresolved binaries and internal variations in helium or metallicity havebeen suggested to explain this phenomenon. Here, we derive high-precisionHubble Space Telescope photometry of the GCs NGC6362 and NGC6838 and buildtheir ChMs. We find that both 1G RGB and main-sequence (MS) stars exhibit widerChM sequences than those of second-generation (2G). The evidence of thisfeature even among unevolved 1G MS stars indicates that chemicalinhomogeneities are imprinted in the original gas. We introduce a pseudotwo-magnitude diagram to distinguish between helium and metallicity, anddemonstrate that star-to-star metallicity variations are responsible for theextended 1G sequence. Conversely, binaries provide a minor contribution to thephenomenon. We estimate that the metallicity variations within 1G stars of 55GCs range from less than [Fe/H]~0.05 to ~0.30 and mildly correlate with clustermass. We exploit these findings to constrain the formation scenarios ofmultiple populations showing that they are qualitatively consistent with theoccurrence of multiple generations. In contrast, the fact that 2G stars havemore homogeneous iron content than the 1G challenges the scenarios based onaccretion of material processed in massive 1G stars onto existing protostars.

Constraining the original composition of the gas forming first-generation stars in globular clusters