The COMBS Survey -- II. Distinguishing the Metal-Poor Bulge from the
  Halo Interlopers
  The metal-poor stars in the bulge are important relics of the Milky Way's
formation history, as simulations predict that they are some of the oldest
stars in the Galaxy. In order to determine if they are truly ancient stars, we
must understand the origins of this population. Currently, it is unclear if the
metal-poor stars in the bulge ([Fe/H] < -1 dex) are merely halo interlopers, a
unique accreted population, part of the boxy/peanut-shaped (B/P) bulge or a
classical bulge population. In this work, we use spectra from the VLT/FLAMES
spectrograph to obtain metallicity estimates using the Ca-II triplet (CaT) of
473 bulge stars (187 of which have [Fe/H]<-1 dex), targeted using SkyMapper
photometry. We also use Gaia DR2 parallaxes and proper motions to infer the
Galactic positions and velocities along with orbital properties for 523 bulge
stars. We employ a probabilistic orbit analysis and find that about half of our
sample has a > 50\% probability of being bound to the bulge, and half are halo
interlopers. We also see that the occurrence rate of halo interlopers increases
steadily with decreasing metallicity across the full range of our sample (-3 <
[Fe/H] < 0.5). Our examination of the kinematics of the confined compared to
the unbound stars indicates the metal-poor bulge ([Fe/H] < -1 dex) comprises at
least two populations; those confined to the boxy/peanut bulge and halo stars
passing through the inner galaxy. We conclude that an orbital analysis
approach, as we have employed, is important to uncover and understand the
composite nature of the metal-poor stars in the inner region.
