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We introduce new color indices $cn^\prime_{\rm JWL}$ (= $Ca_{\rm CTIO} -Ca_{\rm JWL}$) and $ch_{\rm JWL}$ [=$(JWL43 - b) - (b-y)$], accuratephotometric measures of the CN band at $\lambda$3883 and the CH G band,respectively, in the study of the multiple populations (MPs) in globularclusters (GCs). Our photometric CN--CH relation for a large number of red-giantbranch (RGB) in M5 shows that the evolutions of the CN and CH between the CN-wand CN-s populations are not continuous. Armed with our new color indices, weinvestigate the MPs of NGC 6723, finding the RGB populational number ratio of$n$(CN-w):$n$(CN-s) $\approx$ 35.5:64.5 ($\pm$2.8) with no radial gradient.Similar to other normal GCs with MPs, the helium abundance of the CN-spopulation inferred from the RGB bump magnitude is slightly enhanced by $\DeltaY$ = 0.012 $\pm$ 0.012. Our $cn_{\rm JWL}$ and $cn^\prime_{\rm JWL}$color-magnitude diagrams clearly show the discrete double AGB populations inNGC 6723, whose bright AGB populational number ratio is in marginally agreementwith that of the RGB stars within the statistical errors. Finally, oursynthetic $cn_{\rm JWL}$ index is in good agreement with observations, exceptfor the CN-w asymptotic giant branch (AGB). To mitigate the discrepancy in theCN-w AGB may require a mild nitrogen enhancement and/or a large decrement inthe $^{12}$C/$^{13}$C ratio with respect to the bright RGB.

astrophysical journal/the astrophysical journal

Multiple Stellar Populations of Globular Clusters from Homogeneous  Ca--CN--CH Photometry. V. $cn^\prime_{\rm JWL}$ as a Surrogate $cn_{\rm JWL}$  Index and NGC 6723