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We present a detailed analysis of the upper main sequence of the 1.3 Gyr oldopen cluster Trumpler 20. High accuracy BV photometry combined with the VeryLarge Telescope/FLAMES medium-resolution spectroscopy of 954 stars is essentialto understanding the unusual appearance of the color-magnitude diagram (CMD),initially suggesting multiple populations in Trumpler 20. We show thatdifferential reddening is a dominant contributor to the apparentsplitting/widening of the main-sequence turnoff region. At its extreme, theexcess differential reddening reaches Delta(B-V)=0.1 while the adopted minimumreddening for the cluster is E(B-V)=0.36. A unique sample of measured projectedrotational velocities indicates that stellar rotation is high near themain-sequence turnoff, reaching vsin i=180 km/s. By dividing the uppermain-sequence stars into equal groups of slow and fast rotators, we find thatfast rotators have a marginal blueshift of delta(V-I)=-0.01, corresponding to adifference in the median vsin i of 60 km/s between these subsamples. Weconclude that stellar rotation has an insignificant effect on the morphology ofthe upper main sequence of this intermediate-age open cluster. Trumpler 20appears to contain a single coeval population of stars but there is evidencethat the red clump is extended.

The effects of differential reddening and stellar rotation on the appearance of multiple populations in star clusters: the case of Trumpler 20