Cool stars in the Galactic Center as seen by APOGEE: M giants, AGB stars
  and supergiant stars/candidates
  The Galactic Center region, including the nuclear disk, has until recently
been largely avoided in chemical census studies because of extreme extinction
and stellar crowding. Making use of the latest APOGEE data release (DR16), we
are able for the first time to study cool AGB stars and supergiants in this
region. The stellar parameters of five known AGB stars and one supergiant star
(VR 5-7) show that their location is well above the tip of the RGB.We study
metallicities of 157 M giants situated within 150 pc of the Galactic center
from observations obtained by the APOGEE survey with reliable stellar
parameters from the APOGEE/ASPCAP pipeline making use of the cool star grid
down to 3200 K. Distances, interstellar extinction values, and radial
velocities were checked to confirm that these stars are indeed situated in the
Galactic Center region.
  We detect a clear bimodal structure in the metallicity distribution function,
with a dominant metal-rich peak of [Fe/H] ~ +0.3 dex and a metal-poor peak
around [Fe/H]= -0.5 dex, which is 0.2 dex poorer than Baade's Window. The
alpha-elements Mg, Si, Ca, and O show a similar trend to the Galactic Bulge.
The metal-poor component is enhanced in the alpha-elements, suggesting that
this population could be associated with the classical bulge and a fast
formation scenario. We find a clear signature of a rotating nuclear stellar
disk and a significant fraction of high velocity stars with $\rm v_{gal} >
300\,km/s$; the metal-rich stars show a much higher rotation velocity ($\rm
\sim 200\,km/s$) with respect to the metal-poor stars ($\rm \sim 140\,km/s$).
The chemical abundances as well as the metallicity distribution function
suggest that the nuclear stellar disc and the nuclear star cluster show
distinct chemical signatures and might be formed differently.
