Two Groups of Red Giants with Distinct Chemical Abundances in the Bulge
  Globular Cluster NGC 6553 Through the Eyes of APOGEE
  Multiple populations revealed in globular clusters (GCs) are important
windows to the formation and evolution of these stellar systems. The metal-rich
GCs in the Galactic bulge are an indispensable part of this picture, but the
high optical extinction in this region has prevented extensive research. In
this work, we use the high resolution near-infrared (NIR) spectroscopic data
from APOGEE to study the chemical abundances of NGC 6553, which is one of the
most metal-rich bulge GCs. We identify ten red giants as cluster members using
their positions, radial velocities, iron abundances, and NIR photometry. Our
sample stars show a mean radial velocity of $-0.14\pm5.47$ km s$^{-1}$, and a
mean [Fe/H] of $-0.15\pm 0.05$. We clearly separate two populations of stars in
C and N in this GC for the first time. NGC 6553 is the most metal-rich GC where
the multiple stellar population phenomenon is found until now. Substantial
chemical variations are also found in Na, O, and Al. However, the two
populations show similar Si, Ca, and iron-peak element abundances. Therefore,
we infer that the CNO, NeNa, and MgAl cycles have been activated, but the MgAl
cycle is too weak to show its effect on Mg. Type Ia and Type II supernovae do
not seem to have significantly polluted the second generation stars. Comparing
with other GC studies, NGC 6553 shows similar chemical variations as other
relatively metal-rich GCs. We also confront current GC formation theories with
our results, and suggest possible avenues for improvement in the models.
