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 \\nrgb\\ = 25:75 ($\\pm$3; RGB) and 79:21 ($\\pm$13; AGB). The discrepancy in these number ratios suggests that a significant fraction of the low-mass \\cns\\ stars failed to evolve into the AGB phase. However, unlike previous studies, our results indicate the presence of an extreme \\cns\\ 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 \\cns\\ 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 \\cnw\\ population has a spatially elongated shape with a faster rotation, while the \\cns\\ population shows weak or no net rotation, with spatially symmetric shape, raising important question about the long-term dynamical evolution of the GCs.