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

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: paolo.donati4@unibo.it
² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Dipartimento di Fisica e Astronomia, Università di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
⁴ INAF – Osservatorio Astronomico di Padova, vicolo Osservatorio 5, 35122 Padova, Italy
⁵ Department of Astronomy, Indiana University, Bloomington, IN 47405, USA
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁸ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
⁹ Institute of Theoretical Physics and Astronomy, Vilnius University, Goštauto 12, 01108 Vilnius, Lithuania
¹⁰ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
¹¹ CNRS, LAB, UMR 5804, 33270 Floirac, France
¹² Instituto de Astrofísica de Andalucía, CSIC, Apdo 3004, 18080 Granada, Spain
¹³ Departamento de Astronomía, Universidad de Concepción, 160-C Casilla, Concepción, Chile
¹⁴ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
¹⁵ Astronomy Department, University of Geneva, 1211 Geneva, Switzerland
¹⁶ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁷ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
¹⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁹ Moscow M.V. Lomonosov State University, Sternberg Astronomical Institute, Universitetskij pr., 13, 119992 Moscow, Russia
²⁰ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
²¹ ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italy
²² Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France

Received: 25 October 2013
Accepted: 5 December 2013

Abstract

Trumpler 20 is an old open cluster (OC) located toward the Galactic centre, at about 3 kpc from the Sun and ~7 kpc from the Galactic centre. Its position makes this cluster particularly interesting in the framework of the chemical properties of the Galactic disc because very few old OCs reside in the inner part of the disc. For this reason it has been selected as a cluster target of the Gaia-ESO Survey, and spectra of many stars in the main-sequence and red-clump phases are now available. Moreover, although it has been studied by several authors in the past, no consensus on the evolutionary status of Tr 20 has been reached. The heavy contamination of field stars (the line of sight of Tr 20 crosses the Carina spiral arm) complicates a correct interpretation. Another interesting aspect of the cluster is that it shows a broadened main-sequence turn-off and a prominent and extended red-clump, characteristics that are not easily explained by classical evolutionary models. Exploiting both spectroscopic information from the Gaia-ESO Survey (and the ESO archive) and literature photometry, weobtain a detailed and accurate analysis of the properties of the cluster. We make use of the first accurate metallicity measurement ever obtained from several spectra of red clump stars, and of cluster membership determination using radial velocities. According to the evolutionary models adopted, we find that Tr 20 has an age in the range 1.35−1.66 Gyr, an average reddening E(B − V) in the range 0.31−0.35 mag, and a distance modulus (m − M)₀ between 12.64 and 12.72 mag. The spectroscopic metallicity is [Fe/H] = +0.17 dex. We discuss the structural properties of the object and constrain possible hypotheses for its broadened upper main sequence by estimating the effect of differential reddening and its extended red clump.