A spectroscopic survey of thick disc stars outside the solar neighbourhood^⋆,⋆⋆

¹ Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, Cassiopée UMR 6202, BP 4229, 06304 Nice, France
e-mail: georges.kordopatis@oca.eu
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Johns Hopkins University, Baltimore, MD, USA
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁵ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Casilla 306, Santiago 22, Chile
⁶ Université de Strasbourg, Observatoire Astronomique, Strasbourg, France

Received: 30 May 2011
Accepted: 3 October 2011

Abstract

Context. In the era of large spectroscopic surveys, Galactic archaeology aims to understand the formation and evolution of the Milky Way by means of large datasets. In particular, the kinematic and chemical study of the thick disc can give valuable information on the merging history of the Milky Way.

Aims. Our aim is to detect and characterise the Galactic thick disc chemically and dynamically by analysing F, G, and K stars, whose atmospheres reflect their initial chemical composition.

Methods. We performed a spectroscopic survey of nearly 700 stars probing the Galactic thick disc far from the solar neighbourhood towards the Galactic coordinates (l ~ 277°, b ~ 47°). The derived effective temperatures, surface gravities and overall metallicities were then combined with stellar evolution isochrones, radial velocities and proper motions to derive the distances, kinematics and orbital parameters of the sample stars. The targets belonging to each Galactic component (thin disc, thick disc, halo) were selected either on their kinematics or according to their position above the Galactic plane, and the vertical gradients were also estimated.

Results. We present here atmospheric parameters, distances and kinematics for this sample and a comparison of our kinematic and metallicity distributions with the Besançon model of the Milky Way. The thick disc far from the solar neighbourhood is found to differ only slightly from the thick disc properties as derived in the solar vicinity. For regions where the thick disc dominates (1 ≲ Z ≲ 4 kpc), we measured vertical velocity and metallicity trends of ∂V_φ/∂Z = 19 ± 8 km s^-1 kpc^-1 and ∂ [M/H] /∂Z = −0.14 ± 0.05 dex kpc^-1, respectively. These trends can be explained as a smooth transition between the different Galactic components, although intrinsic gradients could not be excluded. In addition, a correlation ∂V_φ/∂ [M/H]  = −45 ± 12 km s^-1 dex^-1 between the orbital velocity and the metallicity of the thick disc is detected. This gradient is inconsistent with the SDSS photometric survey analysis, which did not detect any such trend, and challenges radial migration models of thick disc formation. Estimations of the scale heights and scale lengths for different metallicity bins of the thick disc result in consistent values, with h_R ~ 3.4 ± 0.7 kpc, and h_Z ~ 694 ± 45 pc, showing no evidence of relics of destroyed massive satellites.