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Trumpler 20 is an old open cluster (OC) located toward the Galactic centre,at about 3 kpc from the Sun and $\sim$7 kpc from the Galactic centre. Itsposition makes this cluster particularly interesting in the framework of thechemical properties of the Galactic disc because very few old OCs reside in theinner part of the disc. For this reason it has been selected as a clustertarget of the Gaia-ESO Survey, and spectra of many stars in the main sequenceand red clump phases are now available. Moreover, although it has been studiedby several authors in the past, no consensus on the evolutionary status of Tr20 has been reached. The heavy contamination of field stars (the line of sightof Tr 20 crosses the Carina spiral arm) complicates a correct interpretation.Another interesting aspect of the cluster is that it shows a broadenedmain-sequence turn-off and a prominent and extended red-clump, characteristicsthat are not easily explained by classical evolutionary models. Exploiting bothspectroscopic information from the Gaia-ESO Survey (and the ESO archive) andliterature photometry, we obtain a detailed and accurate analysis of theproperties of the cluster. We make use of the first accurate metallicitymeasurement ever obtained from several spectra of red clump stars, and ofcluster membership determination using radial velocities. According to theevolutionary models adopted, we find that Tr 20 has an age in the range1.35-1.66 Gyr, an average reddening $E(B-V)$ in the range 0.31-0.35 mag, and adistance modulus $(m-M)_0$ between 12.64 and 12.72 mag. The spectroscopicmetallicity is [Fe/H]=+0.17 dex. We discuss the structural properties of theobject and constrain possible hypotheses for its broadened upper main sequenceby estimating the effect of differential reddening and its extended red clump.

The Gaia-ESO Survey: Reevaluation of the parameters of the open cluster Trumpler 20 using photometry and spectroscopy