2012ApJ...751L...8P

We present a detailed analysis of the upper main sequence of the ∼1.3 Gyr old open cluster Trumpler 20. High-accuracy BVI 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 Δ(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 two equal groups of slow and fast rotators, we find that fast rotators have a marginal blueshift of δ(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.