Signature of a chemical spread in the open cluster M37

Abstract

Recent Gaia photometry of the open cluster M37 has 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 $\Delta \rm [Fe/H] \sim 0.15$ plus a differential reddening across the face of the cluster spanning a total range ΔE(B - V) ~ 0.06, or a spread of the initial helium mass fraction ΔY ~ 0.10 plus a smaller range of reddening ΔE(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.