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