The Gaia-ESO Survey: Stellar content and elemental abundances in the massive cluster NGC 6705^⋆,⋆⋆

¹ Dipartimento di Fisica e Astronomia, Università di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
e-mail: tristan.cantat@oapd.inaf.it
² INAF−Osservatorio Astronomico di Padova, vicolo Osservatorio 5, 35122 Padova, Italy
³ INAF−Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ School of Physics, Astronomy & Maths, University of Hertfordshire, Hatfield, Herts. AL10 9AB, UK
⁵ Thueringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁶ CASU, Institute of Astronomy, Madingley Road, University of Cambridge, CB3 0HA, UK
⁷ IGAM, Institute of Physics, University of Graz, Universitätsplatz. 5, 8010 Graz, Austria
⁸ Departament d’Astronomia i Meteorologia, Universitat de Barcelona, Avda. Diagonal 647, 08028 Barcelona, Spain
⁹ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussel, Belgium
¹⁰ Instituto de Astrofísica de Andalucía, CSIC, Apdo 3004, 18080 Granada, Spain
¹¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹² Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
¹³ INAF−Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
¹⁴ Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁵ Department of Astronomy, Indiana University, Bloomington, IN 47405, USA
¹⁶ Massachusetts Institute of Technology and Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
¹⁷ Institute of Theoretical Physics and Astronomy, Vilnius University, Goštauto 12, 01108 Vilnius, Lithuania
¹⁸ INAF−Astrophysical Observatory of Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁹ Section of Astrophysics, Astronomy and Mechanics, Department of Physics, University of Athens, 15784 Athens, Greece
²⁰ INAF−Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
²¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
²² Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00, Sweden
²³ INAF−Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
²⁴ Departamento de Astronomía, Universidad de Concepción, Casilla 160- C Concepción, Chile
²⁵ Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
²⁶ Spanish Virtual Observatory, Centro de Astrobiología (INTA-CSIC), PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
²⁷ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
²⁸ Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile

Received: 20 March 2014
Accepted: 26 June 2014

Abstract

Context. Chemically inhomogeneous populations are observed in most globular clusters, but not in open clusters. Cluster mass seems to play a key role in the existence of multiple populations.

Aims. Studying the chemical homogeneity of the most massive open clusters is needed to better understand the mechanism of their formation and determine the mass limit under which clusters cannot host multiple populations. Here we studied NGC 6705, which is a young and massive open cluster located towards the inner region of the Milky Way. This cluster is located inside the solar circle. This makes it an important tracer of the inner disk abundance gradient.

Methods. This study makes use of BVI and ri photometry and comparisons with theoretical isochrones to derive the age of NGC 6705. We study the density profile of the cluster and the mass function to infer the cluster mass. Based on abundances of the chemical elements distributed in the first internal data release of the Gaia-ESO Survey, we study elemental ratios and the chemical homogeneity of the red clump stars. Radial velocities enable us to study the rotation and internal kinematics of the cluster.

Results. The estimated ages range from 250 to 316 Myr, depending on the adopted stellar model. Luminosity profiles and mass functions show strong signs of mass segregation. We derive the mass of the cluster from its luminosity function and from the kinematics, finding values between 3700 M_⊙ and 11 000 M_⊙. After selecting the cluster members from their radial velocities, we obtain a metallicity of [Fe/H] = 0.10 ± 0.06 based on 21 candidate members. Moreover, NGC 6705 shows no sign of the typical correlations or anti-correlations between Al, Mg, Si, and Na, which are expected in multiple populations. This is consistent with our cluster mass estimate, which is lower than the required mass limit proposed in the literature to develop multiple populations.