The case of the disappearing CN-strong AGB stars in Galactic globular
  clusters -- Preliminary Results
  A previously reported literature search suggested that the AGB stars in
Galactic globular clusters may be showing different distributions of CN-strong
and CN-weak stars as compared to their RGB stars. In most cases the second
giant branches of GCs appeared to be deficient in stars with strong CN bands.
However the sample sizes of AGB stars at that time were too small to give a
definitive picture. Thus an observing campaign targeting GC AGB stars was
proposed. We now have medium resolution spectral observations of about 250 GC
AGB stars across 9 globular clusters, obtained with the 2dF/AAOmega instrument
on the Anglo-Australian Telescope. In this paper we report some preliminary
results regarding the distributions of CN-strong and CN-weak stars on the two
giant branches of a selection of globular clusters. We find that some GCs show
a total lack of CN-strong stars on the AGB, whilst some show a reduction in
CN-strong stars as compared to the RGB. Standard stellar evolution does not
predict this change in surface abundance between the two giant branches. We
discuss some possible causes of this unexpected phenomenon.
