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 ( CN-s ) = 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 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 .