Two groups of red giants with distinct chemical abundances in the bulge globular cluster NGC 6553 through the eyes of APOGEE

Abstract

Multiple populations revealed in globular clusters (GCs) are important windows to the forma-tion and evolution of these stellar systems. The metal-rich GCs in the Galactic bulge are anindispensable part of this picture, but the high optical extinction in this region has preventedextensive research. In this work, we use the high-resolution near-infrared (NIR) spectroscopicdata from Apache Point Observatory Galactic Evolution Experiment (APOGEE) to study thechemical abundances of NGC 6553, which is one of the most metal-rich bulge GCs. We iden-tify 10 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±5.47 km s−1,and a mean [Fe/H] of−0.15±0.05. We clearly separate two populations of stars in C and Nin this GC for the first time. NGC 6553 is the most metal-rich GC where the multiple stellarpopulation phenomenon is found until now. Substantial chemical variations are also foundin Na, O, and Al. However, the two populations show similar Si, Ca, and iron-peak elementabundances. 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 donot seem to have significantly polluted the second generation stars. Comparing with other GCstudies, NGC 6553 shows similar chemical variations as other relatively metal-rich GCs. Wealso confront current GC formation theories with our results, and suggest possible avenues forimprovement in the models.