Cyanogen in NGC 1851 RGB and AGB Stars: Quadrimodal Distributions
  The Galactic globular cluster NGC 1851 has raised much interest since HST
photometry revealed that it hosts a double subgiant branch. Here we report on
our homogeneous study into the cyanogen (CN) bandstrengths in the RGB
population (17 stars) and AGB population (21 stars) using AAOmega/2dF spectra
with R $\sim 3000$. We discover that NGC 1851 hosts a quadrimodal distribution
of CN bandstrengths in its RGB and AGB populations. This result supports the
merger formation scenario proposed for this cluster, such that the CN
quadrimodality could be explained by the superposition of two `normal' bimodal
populations. A small sample overlap with an abundance catalogue allowed us to
tentatively explore the relationship between our CN populations and a range of
elemental abundances. We found a striking correlation between CN and [O/Na]. We
also found that the four CN peaks may be paired -- the two CN-weaker
populations being associated with low Ba and the two CN-stronger populations
with high Ba. If true then s-process abundances would be a good diagnostic for
disentangling the two original clusters in the merger scenario. More
observations are needed to confirm the quadrimodality, and also the
relationship between the subpopulations. We also report CN results for NGC 288
as a comparison. Our relatively large samples of AGB stars show that both
clusters have a bias towards CN-weak AGB populations.
