Campbell, S. W.2; Yong, D.3; Wylie-de Boer, E. C.4; Stancliffe, R. J.5; Lattanzio, J. C.5; Angelou, G. C.5; D'Orazi, V.5; Martell, S. L.6; Grundahl, F.8; Sneden, C.7
1 Department of Physics and Astronomy, Science and Technology, Aarhus University2 Monash Centre for Astrophysics, P.O. Box 28M, Victoria 3800, Australia firstname.lastname@example.org Research School of Astronomy and Astrophysics, Australian National University, Weston, ACT 2611, Australia email@example.com Research School of Astronomy and Astrophysics, Australian National University, Weston, ACT 2611, Australia5 Monash Centre for Astrophysics, P.O. Box 28M, Victoria 3800, Australia6 Australian Astronomical Observatory, North Ryde, NSW 2113, Australia7 Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712, USA8 Department of Physics and Astronomy, Science and Technology, Aarhus University
The Galactic globular cluster NGC 1851 has raised much interest since Hubble Space Telescope photometry revealed that it hosts a double subgiant branch. Here we report on our homogeneous study into the cyanogen (CN) band strengths in the red giant branch (RGB) population (17 stars) and asymptotic giant branch (AGB) population (21 stars) using AAOmega/2dF spectra with R ~ 3000. We discover that NGC 1851 hosts a quadrimodal distribution of CN band strengths 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 catalog 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 toward CN-weak AGB populations.