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Recent Gaia photometry of the open cluster M37 have disclosed the existenceof an extended main-sequence turn off -- like in Magellanic clusters youngerthan about 2 Gyr -- and a main sequence that is broadened in colour beyond whatis expected from the photometric errors, at magnitudes well below the region ofthe extended turn off, where neither age differences nor rotation rates (thecandidates to explain the extended turn off phenomenon) are expected to play arole. Moreover, not even the contribution of unresolved binaries can fullyexplain the observed broadening. We investigated the reasons behind thisbroadening by making use of synthetic stellar populations and differentialcolour-colour diagrams using a combination of Gaia and Sloan filters. From ouranalysis we have concluded that the observed colour spread in the Gaiacolour-magnitude diagram can be reproduced by a combination of either ametallicity spread Delta[Fe/H] ~ 0.15 plus a differential reddening across theface of the cluster spanning a total range DeltaE (B - V) ~ 0.06, or a spreadof the initial helium mass fraction DeltaY ~ 0.10 plus a smaller range ofreddening DeltaE (B - V) ~ 0.03. High-resolution differential abundancedeterminations of a sizeable sample of cluster stars are necessary to confirmor exclude the presence of a metal abundance spread. Our results raise thepossibility that also individual open clusters, like globular clusters andmassive star clusters, host stars born with different initial chemicalcompositions.

Signature of a chemical spread in the open cluster M37