Cyanogen in NGC 1851 Red Giant Branch and Asymptotic Giant Branch Stars: Quadrimodal Distributions

Abstract

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.

A sequence of nitrogen-rich very red giants in the globular cluster NGC 1851

“…In our study, we presented the first spectroscopic derivation of N abundances of giants in this cluster made for such a large number of stars on the RGB, which nicely supports the results by Villanova et al (2010) and Campbell et al (2012) based on much more limited samples. While we cannot state anything on the total C+N+O sum with not knowing the C abundance of the individual objects, we are able to spectroscopically ascertain that stars are located on the redder sequence in NGC 1851 because they are N-rich.…”

“…There are six stars in common between our analysis and the sample of 17 RGB stars analyzed by Campbell et al (2012). Using the Strömgren photometry available to us for these 6 objects, we see that the two stars with the highest values of δS(3839) (stars 41113 and 32112 in Table 1 of Campbell et al 2012) lie on the redder RGB sequence.…”

“…Using the Strömgren photometry available to us for these 6 objects, we see that the two stars with the highest values of δS(3839) (stars 41113 and 32112 in Table 1 of Campbell et al 2012) lie on the redder RGB sequence. The other four stars (46228, 44414, 47385, and 44939) are located on the blue sequence and all have lower values of δS(3839).…”

“…Villanova et al (2010) found a different N content in the two group of stars with different Ba abundances. Campbell et al (2012) recently studied the CN bandstrengths of a sample of 21 AGB stars and 17 RGB stars in NGC 1851; however, they were selected on the V, B − V color-magnitude diagram, where the red and blue RGB sequence cannot be separated. In their two samples they found a quadrimodal distribution of CN strengths with the more CN-weak populations coupled with low Ba abundances and the more CN-strong peaks of the distributions associated with high Ba values.…”

CNO abundances in the globular clusters NGC 1851 and NGC 6752★

“…of AGB stars have indicated that they may be populated exclusively by Na-poor objects (Campbell et al 2013). Furthermore, the RGB and AGB populations in NGC 1851 exhibit a complex distribution in CN molecular line strengths (Campbell et al 2012). Therefore, we were also interested in studying the CNO content of AGB stars in NGC 1851.…”