In disguise or out of reach: first clues about in-situ and accreted
  stars in the stellar halo of the Milky Way from Gaia DR2
  We investigate the nature of the double color-magnitude sequence observed in
the Gaia DR2 HR diagram of stars with high transverse velocities. The stars in
the reddest-color sequence are likely dominated by the dynamically-hot tail of
the thick disk population. Information from Nissen & Schuster (2010) and from
the APOGEE survey suggests that stars in the blue-color sequence have elemental
abundance patterns that can be explained by this population having a relatively
low star-formation efficiency during its formation. In dynamical and orbital
spaces, such as the `Toomre diagram', the two sequences show a significant
overlap, but with a tendency for stars on the blue-color sequence to dominate
regions with no or retrograde rotation and high total orbital energy. In the
plane defined by the maximal vertical excursion of the orbits versus their
apocenters, stars of both sequences redistribute into discrete wedges. We
conclude that stars which are typically assigned to the halo in the solar
vicinity are actually both accreted stars lying along the blue sequence in the
HR diagram, and the low velocity tail of the old Galactic disk, possibly
dynamically heated by past accretion events. Our results imply that a halo
population formed in situ and responsible for the early chemical enrichment
prior to the formation of the thick disk is yet to be robustly identified, and
that what has been defined as the stars of the in situ stellar halo of the
Galaxy may be in fact fossil records of its last significant merger.
