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 an 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 ∆Y ≈ 0.03 -0.07 are required in the G2. Our results support the idea that M22 formed via a merger of two GCs.

Gaia and Hubble Unveil the Kinematics of Stellar Populations in the Type II Globular Clusters ω Centauri and M22

“…This possibility could explain why the stellar populations of M22 share the similar kinematics. However, it is worth noting that our results do not confirm the conclusion by Lee (2015Lee ( , 2020, who find metal-rich stars of M22 rotate faster than metal-poor stars both in the plane of the sky and along the line of sight.…”

“…Type II GCs exhibit distinctive photometric features, including multimodal subgiant branches (SGBs) in colormagnitude diagrams (CMDs) made with optical filters (e.g., Milone et al 2008;Marino et al 2009;Piotto et al 2012), and multimodal red giant branches (RGBs) and SGBs in the I versus U−I or V versus U−V CMDs, with metal-rich stars populating red RGBs and faint SGBs (e.g., Marino et al 2011;Lee 2015Lee , 2020.…”

Multiple Stellar Populations of Globular Clusters from Homogeneous Ca–CN–CH–NH Photometry. VII. Metal-poor Populations in 47 Tucanae (NGC 104)* †

“…However, the carbon and nitrogen abundance difference between the CN-w MP and CN-s MP cannot be easily explained in the context of the chemical evolution of the disk stars in dwarf galaxies, where the C-N anticorrelation does not appear to exist. Instead, the metal-poor components could be understood by an independent GC system that merged with the more massive metal-rich component as we suggested for M22 and M3 (e.g., Lee 2015Lee , 2020Lee & Sneden 2021). The fraction of the metal-poor stars in our study is about 13%, which can be translated into the total mass of ∼1.0 × 10 4 M e (Baumgardt & Hilker 2018).…”

“…In our previous studies (Lee 2017(Lee , 2018(Lee , 2019a(Lee , 2019c(Lee , 2020Lee & Sneden 2021), we showed that both the cn JWL and ¢ cn JWL indices are very powerful tools to classify MPs in GCs. As we already mentioned, our cn JWL observations were limited to the central part of the cluster, and we decided to use the ¢ cn JWL to classify MPs in 47 Tuc in our current study.…”

Formation of Multiple Populations of M5 (NGC 5904)

“…The nh JWL , cn JWL , and ch JWL are excellent measures of the NH band at λ3360, the CN band at λ3883, and the CH G band at λ4250, respectively, for cool stars (Lee 2017(Lee , 2019b(Lee , 2020Lee & Sneden 2021). We made use of the proper motions from the Gaia EDR3 to select the cluster's membership stars, following the method similar to those used in our previous studies (see, e.g., Lee 2020; Lee & Sneden 2021, and references therein).…”