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Extended main sequence turnoffs in open clusters as seen by Gaia -- II. The enigma of NGC 2509

We investigate the morphology of the colour-magnitude diagram (CMD) of the open cluster NGC 2509 in comparison with other Galactic open clusters of similar age using Gaia photometry. At $\sim900$ Myr Galactic open clusters in our sample all show an extended main sequence turn off (eMSTO) with the exception of NGC 2509, which presents an exceptionally narrow CMD. Our analysis of the Gaia data rules out differential extinction, stellar density, and binaries as a cause for the singular MSTO morphology in this cluster. We interpret this feature as a consequence of the stellar rotation distribution within the cluster and present the analysis with MIST stellar evolution models that include the effect of stellar rotation on which we based our conclusion. In particular, these models point to an unusually narrow range of stellar rotation rates ($Ω/Ω_{\rm{crit,ZAMS}} = [0.4, 0.6]$) within the cluster as the cause of this singular feature in the CMD of NGC 2509. Interestingly, models that do not include rotation are not as good at reproducing the morphology of the observed CMD in this cluster.