The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk^⋆

¹ Institute of Astronomy, University of Cambridge Madingley Road, CB3 0HA, Cambridge, UK
e-mail: mbergema@ast.cam.ac.uk
² Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
³ Institute of Space Sciences (IEEC-CSIC), Campus UAB, Fac. Ciències, Torre C5 parell 2, 08193 Bellaterra, Spain
⁴ Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, The Australian National University, ACT 2611, Australia
⁵ Department of Physics and Astronomy, Division of Astronomy and Space Physics, Ångström laboratory, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁷ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁸ Moscow MV Lomonosov State University, Sternberg Astronomical Institute, 119992 Moscow, Russia
⁹ ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italy
¹⁰ Institute of Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹¹ Astrophysics Group, Research Institute for the Environment, Physical Sciences and Applied Mathematics, Keele University, Keele, Staffordshire ST5 5BG, UK
¹² INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134, Palermo, Italy
¹³ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080, Alicante, Spain
¹⁴ ESA, ESTEC, Keplerlaan 1, PO Box 299, 2200 AG Noordwijk, The Netherlands
¹⁵ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁶ INAF – Padova Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁷ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁸ Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
¹⁹ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
²⁰ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
²¹ Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain
²² Institute of Theoretical Physics and Astronomy, Vilnius University, Goštauto 12, 01108 Vilnius, Lithuania
²³ Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, Porto Alegre, RS, Brazil
²⁴ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Universitá di Catania, via S. Sofia 78, 95123 Catania, Italy

Received: 17 January 2014
Accepted: 20 March 2014

Abstract

We study the relationship between age, metallicity, and α-enhancement of FGK stars in the Galactic disk. The results are based upon the analysis of high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore the limitations of the observed dataset, i.e. the accuracy of stellar parameters and the selection effects that are caused by the photometric target preselection. We find that the colour and magnitude cuts in the survey suppress old metal-rich stars and young metal-poor stars. This suppression may be as high as 97% in some regions of the age-metallicity relationship. The dataset consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr, Galactocentric distances from 6 kpcto 9.5 kpc, and vertical distances from the plane 0 < |Z| < 1.5 kpc. On this basis, we find that i) the observed age-metallicity relation is nearly flat in the range of ages between 0 Gyr and 8 Gyr; ii) at ages older than 9 Gyr, we see a decrease in [Fe/H] and a clear absence of metal-rich stars; this cannot be explained by the survey selection functions; iii) there is a significant scatter of [Fe/H] at any age; and iv) [Mg/Fe] increases with age, but the dispersion of [Mg/Fe] at ages >9 Gyr is not as small as advocated by some other studies. In agreement with earlier work, we find that radial abundance gradients change as a function of vertical distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In addition, we show that the inner disk is not only more α-rich compared to the outer disk, but also older, as traced independently by the ages and Mg abundances of stars.