The inner two degrees of the Milky Way. Evidence of a chemical
  difference between the Galactic Center and the surrounding inner bulge
  stellar populations
  Although there have been numerous studies of chemical abundances in the
Galactic bulge, the central two degrees have been relatively unexplored due to
the heavy and variable interstellar extinction, extreme stellar crowding, and
the presence of complex foreground disk stellar populations. In this paper we
discuss the metallicity distribution function, vertical and radial gradients
and chemical abundances of $\alpha$-elements in the inner two degrees of the
Milky Way, as obtained by recent IR spectroscopic surveys. We use a compilation
of recent measurements of metallicities and $\alpha$-element abundances derived
from medium-high resolution spectroscopy. We compare these metallicities with
low-resolution studies.
  Defining "metal-rich" as stars with $ \rm [Fe/H]>0$, and "metal-poor" as
stars with $\rm [Fe/H]<0$, we find compelling evidence for a higher fraction
($\sim 80\%$) of metal-rich stars in the Galactic Center (GC) compared to the
values (50-60\%) measured in the low latitude fields within the innermost 600
pc. The high fraction of metal-rich stars in the GC region implies a very high
mean metallicity of +0.2 dex, while in the inner 600 pc of the bulge the mean
metallicity is rather homogenous around the solar value. A vertical metallicity
gradient of -0.27 dex/kpc in the inner 600 pc is only measured if the GC is
included, otherwise the distribution is about flat and consistent with no
vertical gradient. In addition to its high stellar density, the Galactic
center/nuclear star cluster is also extreme in hosting high stellar abundances,
compared to the surrounding inner bulge stellar populations; this has
implications for formation scenarios and strengthens the case for the NSC being
a distinct stellar system.
