NGC 104, 6121 & 6809 AGB and RGB stars Na abundance

This is the first time that the AGB stars in NGC 6809 are targeted. We find that NGC 104 and NG 6809 have comparable AGB and RGB Na abundance distributions, while NGC 6121 shows a lack of very Na-rich AGB stars.

Moreover, to investigate the dependence of AGB Na abundance dispersion on GC parameters, we compare the AGB [Na/H] distributions of a total of nine GCs, with five determined by ourselves with homogeneous method and four from literature, covering a wide range of GC parameters. Their Na abundances and multiple populations of AGB stars form complex picture. In some GCs, AGB stars have similar Na abundances and/or second-population fractions as their RGB counterparts, while some GCs do not have Na-rich second-population AGB stars, and various cases exist between the two extremes.