  Recent Gaia photometry of the open cluster M37 have disclosed the existence
of an extended main-sequence turn off -- like in Magellanic clusters younger
than about 2 Gyr -- and a main sequence that is broadened in colour beyond what
is expected from the photometric errors, at magnitudes well below the region of
the extended turn off, where neither age differences nor rotation rates (the
candidates to explain the extended turn off phenomenon) are expected to play a
role. Moreover, not even the contribution of unresolved binaries can fully
explain the observed broadening. We investigated the reasons behind this
broadening by making use of synthetic stellar populations and differential
colour-colour diagrams using a combination of Gaia and Sloan filters. From our
analysis we have concluded that the observed colour spread in the Gaia
colour-magnitude diagram can be reproduced by a combination of either a
metallicity spread Delta[Fe/H] ~ 0.15 plus a differential reddening across the
face of the cluster spanning a total range DeltaE (B - V) ~ 0.06, or a spread
of the initial helium mass fraction DeltaY ~ 0.10 plus a smaller range of
reddening DeltaE (B - V) ~ 0.03. High-resolution differential abundance
determinations of a sizeable sample of cluster stars are necessary to confirm
or exclude the presence of a metal abundance spread. Our results raise the
possibility that also individual open clusters, like globular clusters and
massive star clusters, host stars born with different initial chemical
compositions.
