Multiple Stellar Populations of Globular Clusters from Homogeneous Ca–CN Photometry. III. NGC 6752*

Abstract

We present a multiple stellar population (MSP) study of the globular cluster (GC) NGC 6752. We show that our new photometric CN index accurately traces the CN and the nitrogen abundances in cool giants, finding the discrete double red giant branch (RGB) and asymptotic giant branch (AGB) sequences with number ratios between the CN-weak and the CN-strong populations of n(CN-w):n(CNs) = 25:75 (±3; RGB) and 79:21 (±13; AGB). The discrepancy in these number ratios suggests that a significant fraction of the low-mass CN-s stars failed to evolve into the AGB phase. However, unlike previous studies, our results indicate the presence of an extreme CN-s AGB population in NGC 6752, which may require follow-up spectroscopic study. Similar to what is seen for M5, the evolution of the nitrogen abundance is discrete and discontinuous, while the evolutions of oxygen and sodium are continuous between the two populations in NGC 6752, implying that different astrophysical sources are responsible for the evolutions of these elements. In addition, the helium abundance inferred from the RGB bump magnitude shows hat the CN-s population is slightly more helium-enhanced. Despite the identical cumulative radial distributions between the two populations, the structure-kinematics coupling can be observed in individual populations: the CN-w population has a spatially elongated shape with a faster rotation, while the CN-s population shows weak or no net rotation, with spatially symmetric shape, raising important question about the long-term dynamical evolution of the GCs. Based on observations made with the Cerro Tololo Inter-American Observatory (CTIO) 1 m telescope, which is operated by the SMARTS consortium.

Five Stellar Populations in M22 (NGC 6656)

“…The G2 appears to have larger projected tangential velocities than the G1 does. mediate to high total masses, ≈ 0.30 (e.g., see Lee 2017Lee , 2018Lee , 2019aMilone et al 2017). We note that the populational characteristic of the M22 G1 group (the subpopulational number ratio and the CRDs with a strong radial gradient as will be discussed below) is very similar to that of M3 (Lee 2019a).…”

“…It is worth noting the absence of any clear subpopulational cn JWL separations in the G1 (dark green) and G2 (purple) in M22, as shown in Figure 2(b), which is in sharp contrast to the normal GCs without metallicity spread (e.g. M3, M5, NGC 6723, and NGC 6752) exhibiting discrete double cn JWL or cn JWL RGB sequences in our previous studies (e.g., see Lee 2017Lee , 2018Lee , 2019a. Instead, the G1 and G2 distributions projected onto the ∆ 2 axis with the slope of −1 (i.e., on the line along the CN-CH anticorrelation) exhibit the double and triple peaks, respectively.…”

“…The ∆ 2 distribution of the G1 (i.e., the lower-metallicity group) can be fitted best with a two-component model without a helium enhancement between the two subpopulations, namely, the G1 CN-w and CN-s, inferred from their RGBB magnitudes, which is in sharp contrast to normal GCs with significant helium enhancements between the CN-w and CNs populations (e.g., see Lee 2017Lee , 2018Lee , 2019bLagioia et al 2018;Milone et al 2018). On the other hand, the ∆ 2 distribution of the G2 (i.e., the higher-metallicity group) can be fitted best with three subpopulations, namely, the G2 CN-w, CN-i, and CN-s.…”

“…The CRDs of individual populations in GCs may provide a crucial information on the long-term dynamical evolution of GCs (e.g., see Vesperini et al 2013). For normal GCs without any perceptible metallicity spread, the CRDs of the CN-w and CN-s populations in M5, NGC 6723, and NGC 6752 are very similar and statistical tests suggest that their CN-w and CN-s populations are most likely drawn from same parent distributions (Lee 2017(Lee , 2018(Lee , 2019b. On the other hand, the CN-s population in M3 shows a more centrally concentrated CRD (Lee 2019a).…”

“…Figure 5 and Table 5 show our results. The G1 CN-w and CN-s have almost the same RGBB V magnitudes, 2 a strong observational line of evidence that both subpopulations have the same metallicity and helium abundance, in sharp contrast to normal GCs, such as M5, NGC 6723, and NGC 6752, with discernible helium enhancements in their CN-s populations (Lee 2017(Lee , 2018(Lee , 2019b. Our result poses a strong constraint on the polluter of the chemical evolution of the G1 group: no helium enhancement but variations in C and N. Furthermore, the extent of the C and N variations in the G1 group is smaller than that in the G2 group.…”