Multiple stellar populations of globular clusters from homogeneous Ca-Cn photometry. II. M5 (NGC 5904) and a new filter system

Abstract

Using our ingeniously designed new filter systems, we investigate the multiple stellar populations of the RGB and AGB in the GC M5. Our results are the following. (1) Our cn_jwl index accurately traces the nitrogen abundances in M5, while other color indices fail to do so. (2) We find bimodal CN distributions both in the RGB and the AGB sequences, with the number ratios between the CN-weak (CN-w) and the CN-strong (CN-s) of n(CN-w):n(CN-s) = 29:71(+/- 2) and 21:79(+/- 7), respectively. (3) We also find the bimodal photometric [N/Fe] distribution for M5 RGB stars. (4) Our cn_jwl-[O/Fe] and cn_jwl-[Na/Fe] relations show the clear discontinuities between the two RGB populations. (5) Although small, the RGB bump of the CN-s is slightly brighter, 0.07 +/- 0.04 mag. If real, the difference in the helium abundance becomes \dy\=0.028$\pm$0.016, in the sense that the CN-s is more helium enhanced. (6) Very similar radial but different spatial distributions with comparable center positions are found for the two RGB populations. The CN-s RGB and AGB stars are more elongated along the NW-SE direction. (7) The CN-s population shows a substantial net projected rotation, while that of the CN-w population is nil. (8) Our results confirm the deficiency of the CN-w AGB stars previously noted by others. We show that it is most likely due to the stochastic truncation in the outer part of the cluster. Finally, we discuss the formation scenario of M5.