The Gaia-ESO Survey: Churning through the Milky Way

¹ Laboratoire Lagrange (UMR 7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
e-mail: mhayden@oca.eu
² Institute of Theoretical Physics and Astronomy, Vilnius University, Saulėtekio al. 3, 10257 Vilnius, Lithuania
³ Institute of Astronomy, Cambridge University, Madingley Road, Cambridge CB3 0HA, UK
⁴ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaśo, Av. Gran Bretaña 1111, 5030 Casilla, Valparaíso, Chile
⁶ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
⁸ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁹ Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain
¹⁰ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹¹ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Universitá di Catania, via S. Sofia 78, 95123 Catania, Italy
¹² INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
¹³ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁴ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
¹⁵ Departamento de Ciencias Fisicas, Universidad Andres Bello, Republica 220, Santiago, Chile
¹⁶ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia

Received: 10 January 2017
Accepted: 5 June 2017

Abstract

Context. There have been conflicting results with respect to the extent that radial migration has played in the evolution of the Galaxy. Additionally, observations of the solar neighborhood have shown evidence of a merger in the past history of the Milky Way that drives enhanced radial migration.

Aims. We attempt to determine the relative fraction of stars that have undergone significant radial migration by studying the orbital properties of metal-rich ([Fe/H] > 0.1) stars within 2 kpc of the Sun. We also aim to investigate the kinematic properties, such as velocity dispersion and orbital parameters, of stellar populations near the Sun as a function of [Mg/Fe] and [Fe/H], which could show evidence of a major merger in the past history of the Milky Way.

Methods. We used a sample of more than 3000 stars selected from the fourth internal data release of the Gaia-ESO Survey. We used the stellar parameters from the Gaia-ESO Survey along with proper motions from PPMXL to determine distances, kinematics, and orbital properties for these stars to analyze the chemodynamic properties of stellar populations near the Sun.

Results. Analyzing the kinematics of the most metal-rich stars ([Fe/H] > 0.1), we find that more than half have small eccentricities (e< 0.2) or are on nearly circular orbits. Slightly more than 20% of the metal-rich stars have perigalacticons R_p> 7 kpc. We find that the highest [Mg/Fe], metal-poor populations have lower vertical and radial velocity dispersions compared to lower [Mg/Fe] populations of similar metallicity by ~10 km s^-1. The median eccentricity increases linearly with [Mg/Fe] across all metallicities, while the perigalacticon decreases with increasing [Mg/Fe] for all metallicities. Finally, the most [Mg/Fe]-rich stars are found to have significant asymmetric drift and rotate more than 40 km s^-1 slower than stars with lower [Mg/Fe] ratios.

Conclusions. While our results cannot constrain how far stars have migrated, we propose that migration processes are likely to have played an important role in the evolution of the Milky Way, with metal-rich stars migrating from the inner disk toward to solar neighborhood and past mergers potentially driving enhanced migration of older stellar populations in the disk.