The Gaia-ESO Survey: Churning through the Milky Way
  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 using a sample of more than 3,000
stars selected from iDR4 of the Gaia-ESO Survey. We 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. This was done
using 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. 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 $\sim10$ 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. 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.
