The effects of differential reddening and stellar rotation on the   appearance of multiple populations in star clusters: the case of Trumpler 20
  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. 
I. Platais (1), C. Melo (2), S. N. Quinn (3), J. L. Clem (4), S. E. de   Mink (5), A. Dotter (5), V. Kozhurina-Platais (5), ...
astro-ph.SR