Are There Radical Cyanogen Abundance Differences Between Galactic   Globular Cluster RGB and AGB Stars?
  [Abridged] Norris et al. (1981) found that there was a distinct lack of cyanogen-strong (CN-strong) stars in their sample of AGB stars, as compared to their sample of RGB stars (which had roughly equal numbers of CN-normal and CNstrong stars). Similar features have been discovered in the AGB populations of other clusters. Unfortunately all of these studies suffer from low AGB star counts so the conclusions are not necessarily robust -- larger, statistically signifcant, sample sizes are needed.   We outline the results of a literature search for relevant CN observations and describe our observing proposal to test the suggestion that there are substantial abundance differences between the AGB and RGB in galactic globular clusters. The literature search revealed that the AGB star counts for all studies (which are not, in general, studies about AGB stars in particular) are low, usually being < or = 10. The search also revealed that the picture may not be consistent between clusters. Although most clusters appear to have CN-weak AGBs, at least two seem to have CN-strong AGBs (M5 & 47 Tuc). To further complicate the picture, clusters often appear to have a combination of both CN-strong and CN-weak stars on their AGBs -- although one population tends to dominate. Again, all these assertions are however based on small sample sizes.We aim to increase the sample sizes by an order of magnitude using existing high quality photometry in which the AGB and RGB can be reliably separated. For the observations we will use a wide-field, low- to mid-resolution multi-object spectroscope. With the new information we hope to ascertain whether significant abundance differences really exist. 
S. W. Campbell, J. C. Lattanzio and L. M. Elliott (Centre for Stellar   and Planetary Astrophysics, Monash University, Melbo...