The Gaia-ESO Survey: Insights into the inner-disc evolution from open clusters^⋆

¹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
e-mail: laura@arcetri.astro.it
² INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Dipartimento di Fisica e Astronomia, Universitá di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
⁶ Observatoire de Genève, Université de Genève, 1290 Versoix, Switzerland
⁷ Institute of Theoretical Physics and Astronomy, Vilnius University, Gostauto 12, 01108 Vilnius, Lithuania
⁸ Astronomy Department, Indiana University, 727 East 3rd Street, Bloomington, IN 47405, USA
⁹ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
¹⁰ Dipartimento di Fisica e Astronomia, Universitá di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
¹¹ INAF–Osservatorio Astronomico di Padova, vicolo Osservatorio 5, 35122 Padova, Italy
¹² ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italy
¹³ Departamento de Astronomía, Casilla 160-C, Universidad de Concepción, Concepción, Chile
¹⁴ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, 44001 Teruel, Spain
¹⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁶ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹⁷ Dept. of Astronomy and Theoretical physics, Lund university, Box 43, 22100 Lund, Sweden
¹⁸ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134, Palermo, Italy
¹⁹ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
²⁰ Laboratoire Lagrange (UMR 7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
²¹ European Southern Observatory, Alonso de Cordova 3107 Vitacura, Santiago de Chile, Chile
²² Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain
²³ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁴ Departamento de Ciencias Físicas, Universidad Andrés Bello, República 220, 837-0134 Santiago, Chile
²⁵ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

Received: 13 April 2015
Accepted: 13 May 2015

Abstract

Context. The inner disc, which links the thin disc with the bulge, has been somewhat neglected in the past because of the intrinsic difficulties in its study, among which crowding and high extinction. Open clusters located in the inner disc are among thebest tracers of its chemistry at different ages and distances.

Aims. We analyse the chemical patterns of four open clusters located within 7 kpc of the Galactic centre and of field stars to infer the properties of the inner disc with the Gaia-ESO survey idr2/3 data release.

Methods. We derive the parameters of the newly observed cluster, Berkeley 81, finding an age of about 1 Gyr and a Galactocentric distance of ~5.4 kpc. We construct the chemical patterns of clusters and we compare them with those of field stars in the solar neighbourhood and in the inner-disc samples.

Results. Comparing the three populations we observe that inner-disc clusters and field stars are both, on average, enhanced in [O/Fe], [Mg/Fe], and [Si/Fe]. Using the idr2/3 results of M67, we estimate the non-local thermodynamic equilibrium (NLTE) effect on the abundances of Mg and Si in giant stars. After empirically correcting for NLTE effects, we note that NGC 6705 and Be 81 still have a high [α/Fe].

Conclusions. The location of the four open clusters and of the field population reveals that the evolution of the metallicity [Fe/H] and of [α/Fe] can be explained within the framework of a simple chemical evolution model: both [Fe/H] and [α/Fe] of Trumpler 20 and of NGC 4815 are in agreement with expectations from a simple chemical evolution model. On the other hand, NGC 6705, and to a lesser degree Berkeley 81, have higher [α/Fe] than expected for their ages, location in the disc, and metallicity. These differences might originate from local enrichment processes as explained in the inhomogeneous evolution framework.