标题： NGC 1851红巨星和渐近巨星分支星中的氰化物：四模分布 显示英文标题

标题： Cyanogen in NGC 1851 RGB and AGB Stars: Quadrimodal Distributions

摘要： 银河系球状星团NGC 1851自哈勃太空望远镜（HST）光度测量揭示其拥有双次巨星分支以来引起了广泛关注。 在此，我们报告了利用AAOmega/2dF光谱（R$\sim 3000$）对RGB种群（17颗恒星）和AGB种群（21颗恒星）中的氰化物（CN）带强度进行的同质研究。 我们发现NGC 1851在其RGB和AGB种群中具有四模态的CN带强度分布。 这一结果支持了该星团提出的合并形成情景，即CN四模态可通过两个“正常”的双模态种群的叠加来解释。 一个小样本与丰度目录的重叠使我们能够初步探讨我们的CN种群与一系列元素丰度之间的关系。 我们发现CN与[O/Na]之间存在显著的相关性。 我们还发现四个CN峰值可能是成对的——两个CN较弱的种群与低Ba相关，而两个CN较强的种群与高Ba相关。 如果属实，则s过程的丰度可以作为在合并情景中区分两个原始星团的良好诊断工具。 需要更多的观测来确认四模态性以及子种群之间的关系。 我们还报告了NGC 288的CN结果作为比较。 我们相对较大的AGB星体样本显示，这两个星团都偏向于CN弱的AGB种群。 显示英文摘要

摘要： 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.