New sub-grouping of multiple stellar populations in NGC 2808 based on low-resolution spectroscopy

TL;DR

This study uses low-resolution spectroscopy to identify and analyze four distinct subpopulations within the globular cluster NGC 2808, revealing differences in chemical abundances and demonstrating the value of combining spectroscopic methods.

Contribution

It introduces a new sub-grouping scheme for NGC 2808's stellar populations based on CN, CH, and Ca spectral indices, enhancing understanding of their chemical differences.

Findings

Four subpopulations identified with distinct spectral signatures.

CN index more effectively separates early subpopulations than high-resolution data.

G4 shows stronger Ca II H&K lines, possibly due to He enhancement or Mg depletion.

Abstract

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.