The Gaia-ESO Survey: the Galactic thick to thin disc transition^⋆,⋆⋆

¹ Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
e-mail: arecio@oca.eu
² Institute of Astronomy, Cambridge University, Madingley Road, Cambridge CB3 0HA, UK
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Johns Hopkins University, Homewood Campus, 3400 N Charles Street, Baltimore, MD 21218, USA
⁵ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
⁷ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁸ Dept. of Astronomy and Theoretical physics, Lund university, Box 43, 22100 Lund, Sweden
⁹ Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK
¹⁰ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹¹ Osservatorio Astronomico di Padova, Vicolo dell-Osservatorio 5, 35122 Padova, Italy
¹² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía, 18008 - Granada, Spain
¹³ Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain
¹⁴ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, Bologna, Italy
¹⁵ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
¹⁶ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Universitá di Catania, via S. Sofia 78, 95123, Catania, Italy
¹⁷ ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italia
¹⁸ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁹ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
²⁰ Moscow M.V. Lomonosov State University, Sternberg Astronomical Institute, Universitetskij pr., 13, 119992 Moscow, Russia

Received: 29 October 2013
Accepted: 28 March 2014

Abstract

Aims. The nature of the thick disc and its relation to the thin disc is presently an important subject of debate. In fact, the structural and chemo-dynamical transition between disc populations can be used as a test of the proposed models of Galactic disc formation and evolution.

Methods. We used the atmospheric parameters, [α/Fe] abundances, and radial velocities, which were determined from the Gaia-ESO Survey GIRAFFE spectra of FGK-type stars (first nine months of observations) to provide a chemo-kinematical characterisation of the disc stellar populations. We focussed on a subsample of 1016 stars with high-quality parameters, covering the volume | Z | < 4.5 kpc and R in the range 2–13 kpc.

Results. We have identified a thin to thick disc separation in the [α/Fe] vs. [M/H] plane, thanks to the presence of a low-density region in the number density distribution. The thick disc stars seem to lie in progressively thinner layers above the Galactic plane, as metallicity increases and [α/Fe] decreases. In contrast, the thin disc population presents a constant value of the mean distance to the Galactic plane at all metallicities. In addition, our data confirm the already known correlations between V_φ and [M/H] for the two discs. For the thick disc sequence, a study of the possible contamination by thin disc stars suggests a gradient up to 64 ± 9 km s^-1 dex^-1. The distributions of azimuthal velocity, vertical velocity, and orbital parameters are also analysed for the chemically separated samples. Concerning the gradients with galactocentric radius, we find, for the thin disc, a flat behaviour of the azimuthal velocity, a metallicity gradient equal to −0.058 ± 0.008 dex kpc^-1 and a very small positive [α/Fe] gradient. For the thick disc, flat gradients in [M/H] and [α/Fe] are derived.

Conclusions. Our chemo-kinematical analysis suggests a picture where the thick disc seems to have experienced a settling process, during which its rotation increased progressively and, possibly, the azimuthal velocity dispersion decreased. At [M/H] ≈ −0.25 dex and [α/Fe]≈ 0.1 dex, the mean characteristics of the thick disc in vertical distance to the Galactic plane, rotation, rotational dispersion, and stellar orbits’ eccentricity agree with that of the thin disc stars of the same metallicity, suggesting a possible connection between these two populations at a certain epoch of the disc evolution. Finally, the results presented here, based only on the first months of the Gaia ESO Survey observations, confirm how crucial large high-resolution spectroscopic surveys outside the solar neighbourhood are today for our understanding of the Milky Way history.