Photometric characterization of the Galactic star cluster Trumpler 20
  We present deep UBVI photometry for Trumpler 20, a rich, intermediate-age
open cluster located at l=301.47, b=+2.22 (RA=12:39:34, DEC=-60:37:00, J2000.0)
in the fourth Galactic quadrant. In spite of its interesting properties, this
cluster has received little attenti on, probably because the line of sight to
it crosses twice the Carina spiral arm, which causes a significant
contamination of its color-magnitude diagram (CMD) by field stars, therefore
complicating seriously its interpretation. We provide more robust estimates of
the fundamental parameters of Trumpler 20, and investigate the most prominent
features of its CMD: a rich He-burning star clump, and a vertical sequence of
stars above the turnoff, which can be either blue stragglers or field stars.
Our precise photometry has allowed us to derive updated values of the age and
heliocentric distance of Trumpler 20, which we estimate to be 1.4 $\pm$ 0.2 Gyr
and 3.0 $\pm$ 0.3 kpc, respectively. As predicted by models, at this age the
clump has a tail towards fainter magnitudes and bluer colors, thus providing
further confirmation of the evolutionary status of stars in this particular
phase. The derived heliocentric distance places the cluster in the inter-arm
region between the Carina and Scutum arms, which naturally explains the
presence of the vertical sequence of stars (which was originally interpreted as
the cluster itself) observed in the upper part of the CMD.Most of these stars
would therefore belong to the general galactic field, while only a few of
themwould be bona fide cluster blue stragglers. Our data suggest that the
cluster metallicity is solar, and that its reddening is \textit{E(B-V)} = 0.35
$\pm$ 0.04.
