Searching for a kinematic signature of the moderately metal-poor stars
  in the Milky Way bulge using N-body simulations
  Although there is consensus that metal-rich stars in the Milky Way bulge are
formed via secular evolution of the thin disc, the origin of their metal-poor
counterparts is still under debate. Two different origins have been invoked for
metal-poor stars: they might be classical bulge stars or stars formed via
internal evolution of a massive thick disc. We use N-body simulations to
calculate the kinematic signature given by the difference in the mean
Galactocentric radial velocity ($\Delta\rm V_{\rm GC}$) between metal-rich
stars ([Fe/H] $\ge$ 0) and moderately metal-poor stars (-1.0 $\le$ [Fe/H] $<$
0) in two models, one containing a thin disc and a small classical bulge
(B/D=0.1), and the other containing a thin disc and a massive centrally
concentrated thick disc. We reasonably assume that thin-disk stars in each
model may be considered as a proxy of metal-rich stars. Similarly, bulge stars
and thick-disc stars may be considered as a proxy of metal-poor stars. We
calculate $\Delta\rm V_{\rm GC}$ at different latitudes ($b=0^\circ$,
$-2^\circ$, $-4^\circ$,$-6^\circ$, $-8^\circ$ and $-10^\circ$) and longitudes
($l=0^\circ$, $\pm5^\circ$, $\pm10^\circ$ and $\pm15^\circ$) and show that the
$\Delta\rm V_{\rm GC}$ trends predicted by the two models are different. We
compare the predicted results with ARGOS data and APOGEE DR13 data and show
that moderately metal-poor stars are well reproduced with the co-spatial
stellar discs model, which has a massive thick disc. Our results give more
evidence against the scenario that most of the metal-poor stars are classical
bulge stars. If classical bulge stars exists, most of them probably have
metallicities [Fe/H] $<$ -1 dex, and their contribution to the mass of the
bulge should be a small percentage of the total bulge mass.
