We present deep UBVI photometry for Trumpler 20, a rich, intermediate-age
open cluster located at
l = 301.47o, b = +2.22o (α = 12h39m34s,
δ = -60d37m00s, 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 (and possibly also the Scutum-Crux arm), which causes a significant
contamination of its color-magnitude diagram (CMD) by field stars, therefore
complicating seriously its interpretation.
In this paper 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, in combination with previous investigations, has allowed
us to
derive updated values of the age and heliocentric distance of Trumpler 20, which
we estimate
to be 1.4±0.2 Gyr and 3.0±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
them
would be bona fide
cluster blue stragglers.
Our data suggest that the cluster metallicity is solar, and that its reddening is
E(B-V)
= 0.35±0.04.
Finally, we believe we have solved a previously reported inconsistency between
the spectroscopic temperatures and colors of giant stars in the cluster.