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We investigate the nature of the double color-magnitude sequence observed inthe Gaia DR2 HR diagram of stars with high transverse velocities. The stars inthe reddest-color sequence are likely dominated by the dynamically-hot tail ofthe thick disk population. Information from Nissen & Schuster (2010) and fromthe APOGEE survey suggests that stars in the blue-color sequence have elementalabundance patterns that can be explained by this population having a relativelylow star-formation efficiency during its formation. In dynamical and orbitalspaces, such as the `Toomre diagram', the two sequences show a significantoverlap, but with a tendency for stars on the blue-color sequence to dominateregions with no or retrograde rotation and high total orbital energy. In theplane defined by the maximal vertical excursion of the orbits versus theirapocenters, stars of both sequences redistribute into discrete wedges. Weconclude that stars which are typically assigned to the halo in the solarvicinity are actually both accreted stars lying along the blue sequence in theHR diagram, and the low velocity tail of the old Galactic disk, possiblydynamically heated by past accretion events. Our results imply that a halopopulation formed in situ and responsible for the early chemical enrichmentprior to the formation of the thick disk is yet to be robustly identified, andthat what has been defined as the stars of the in situ stellar halo of theGalaxy may be in fact fossil records of its last significant merger.

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