The asymptotic giant branches of GCs: selective entry only

The limited observations of AGB stars in Galactic Globular Clusters (GCs) suggest that the GC AGB populations are dominated by cyanogen (CN) weak stars. This contrasts with the distributions in the RGB (and other) populations, which generally show a 50:50 bimodality in CN. If it is true that the AGB populations show different distributions then it presents a problem for low mass stellar evolution theory, since such a surface abundance change going from the RGB to AGB is not predicted by models. However this is only a tentative conclusion, since it is based on small AGB samples. To test whether this problem is real we have carried out an observational campaign targeting AGB stars in GCs. We obtained medium resolution spectra for 250 AGB stars across 9 GCs using 2df/AAOmega on the AAT. We present some preliminary findings for the second parameter trio of GCs: NGC 288, NGC 362 and NGC 1851. The results indeed show that there is a deficiency of stars with strong CN bands on the AGB. To confirm that this phenomenon is robust and not just confined to CN band strengths and their vagaries, we have made observations using FLAMES/VLT to measure elemental abundances for NGC 6752. Our VLT sodium abundance results show conclusively that only a subset of stars in GCs experience the AGB phase of evolution. This is the first concrete confirmation of the phenomenon.