We present deep UBVI photometry for Trumpler 20 , a rich , intermediate-age open cluster located at l = 301.47 \arcdeg , b = +2.22 \arcdeg ( \alpha = 12 ^ { h } ~ { } 39 ^ { m } ~ { } 34 ^ { s } , \delta = -60 \arcdeg~ { } 37 ^ { \prime } ~ { } 00 ^ { \prime \prime } , J2000.0 ) in the fourth Galactic quadrant .
In spite of its interesting properties , this cluster has received little attention , 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 \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 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 \pm 0.04 .
Finally , we believe we have solved a previously reported inconsistency between the spectroscopic temperatures and colors of giant stars in the cluster .