Vol. 583
In section 5. Galactic structure, stellar clusters and populations

The Gaia-ESO Survey: New constraints on the Galactic disc velocity dispersion and its chemical dependencies

Understanding the formation of the Galactic disk is a key problem in modern astrophysics. Coupling chemical abundances and kinematics for large samples of individual stars is well established as a powerful tool for studying the stellar populations of the Milky Way. The Gaia-ESO survey will provide velocity and abundance measurements from the ESO FLAMES/GIRAFFE spectrograph for many thousands of stars in Galactic disk. In this paper the first new constraints are presented, based on 7800 FGK stars from the second data release. The authors present the velocity dispersion of the Galactic disk stars as a function of [Mg/Fe], which for small metallicity bins can be used as a proxy of relative age. The large survey volume covers, around the solar neighborhood, the range 6-10 kpc from the center of the Galaxy in the plane of the disk and +/-2 kpc out of the plane. The thin and thick disks are clearly separated chemically, and a weak positive correlation is found between Vφ and [Fe/H] for the thin disk, due to the presence of a slowly rotating iron-poor tail. For the thick disk, a strong correlation with [Fe/H] and [Mg/Fe] is established. The results confirm, for a larger volume, the existence of [Mg/Fe]-rich thick-disk stars with cool kinematics in the generally turbulent context of the primitive Galactic disk. This result places significant constraints on disk-formation and evolution scenarios.