Gaia reveals a metal-rich in-situ component of the local stellar halo
  We use the first Gaia data release, combined with RAVE and APOGEE
spectroscopic surveys, to investigate the origin of halo stars within <~3 kpc
from the Sun. We identify halo stars kinematically, as moving with a relative
speed of at least 220 km/s with respect to the local standard of rest. These
stars are in general more metal-poor than the disk, but surprisingly, half of
our halo sample is comprised of stars with [Fe/H]>-1. The orbital directions of
these metal-rich halo stars are preferentially aligned with the disk rotation,
in sharp contrast with the isotropic orbital distribution of the more
metal-poor halo stars. We find similar properties in the Latte cosmological
zoom-in simulation of a Milky Way-like galaxy from the FIRE project. In Latte,
metal-rich halo stars formed primarily inside of the solar circle, while
lower-metallicity halo stars preferentially formed at larger distances
(extending beyond the virial radius). This suggests that metal-rich halo stars
in the Solar neighborhood in fact formed in-situ within the Galactic disk
rather than having been accreted from satellite systems. These stars, currently
on halo-like orbits, therefore have likely undergone substantial radial
migration/heating.
