We performed low-resolution spectroscopy for the red giant branch stars in an intriguing globular cluster (GC) NGC 2808, which hosts subpopulations with extreme helium and light-element abundances. In order to trace N, C, and Ca abundance differences among subpopulations, we measured CN, CH, and Ca ii H&K spectral indices, respectively. We identified four subpopulations (G1, G2, G3, and G4) from CN and CH strength, with CN-weak/CH-strong G1, CN-intermediate/CH-strong G2, CN-strong/CH-intermediate G3, and CN-strong/CH-weak G4. Compared to [Na/O] from high-resolution spectroscopy, we show that CN index can more clearly separate G1 and G2. Since CN traces N abundance in a GC, it implies that G1 and G2 would show a larger difference in [N/Fe] compared to [Na/Fe], as predicted by chemical evolution models. Later generation stars G3 and G4, however, are better separated with high-resolution spectroscopy. We also found that G4 shows a stronger Ca ii H&K line strength compared to that of G1, but we suspect this to be a result of unusually strong He enhancement and/or Mg depletion in G4 of this GC. This work illustrates that combining low- and high-resolution spectroscopic studies can improve the separation of subpopulations in GCs.

我们对一个有趣的球状星团 (GC) NGC 2808 中的红巨星分支星进行了低分辨率光谱分析，该星团拥有极端氦和轻元素丰度的亚群。为了追踪亚群之间的 N、C 和 Ca 丰度差异，我们测量了 CN、CH 和 Ca ii分别为 H&K 谱指数。我们从 CN 和 CH 强度中确定了四个亚群（G1、G2、G3 和 G4），分别是 CN-弱/CH-强 G1、CN-中间/CH-强 G2、CN-强/CH-中间 G3 和 CN -强/CH-弱G4。与来自高分辨率光谱的 [Na/O] 相比，我们表明 CN 指数可以更清楚地分离 G1 和 G2。由于 CN 在 GC 中追踪 N 丰度，这意味着 G1 和 G2 与 [Na/Fe] 相比，[N/Fe] 的差异更大，正如化学演化模型所预测的那样。然而，较晚的一代恒星 G3 和 G4 可以通过高分辨率光谱更好地分离。我们还发现 G4 显示出更强的 Ca ii与 G1 相比，H&K 线强度，但我们怀疑这是由于此 GC 的 G4 中异常强烈的 He 增强和/或 Mg 消耗所致。这项工作表明，结合低分辨率和高分辨率光谱研究可以改善 GC 中亚群的分离。