Five Stellar Populations in M22 (NGC 6656)

Abstract

We present the Ca–CN–CH photometry of the metal-complex globular cluster (GC) M22 (NGC 6656). Our photometry clearly shows the discrete double CN–CH anticorrelations in M22 red giant branch (RGB) stars, due to the difference in the mean metallicity. The populational number ratio between the two main groups is n (G1): n (G2) = 63:37(±3), with the G1 being more metal-poor. Furthermore, the G1 can be divided into two subpopulations with the number ratio of n (CN-w): n (CN-s) = 51:49 (±4), while the G2 can be divided into three subpopulations with n (CN-w): n (CN-i): n (CN-s) = 24:32:44 (±5). The proper motion of individual stars in the cluster shows evidence of internal rotation, showing the G2 with a faster rotation, confirming our previous results from radial velocities. The cumulative radial distributions (CRDs) of individual subpopulations are intriguing in the following aspects: (1) In both main groups, the CRDs of the CN-s subpopulations are more centrally concentrated than other subpopulations. (2) The CRDs of the the G1 CN-s and the G2 CN-s are very similar. (3) Likewise, the G1 CN-w and the G2 CN-w and CN-i have almost identical CRDs. We also estimate the relative helium abundance of individual subpopulations by comparing their RGB bump magnitudes, finding that no helium abundance variation can be seen in the G1, while significant helium enhancements by  ≈ 0.03–0.07 are required in the G2. Our results support the idea that M22 formed via a merger of two GCs.