Comparing CN and CH line strengths in a homogeneous spectroscopic sample of 8 Galactic globular clusters

Our work focuses on the understanding of the origin of CNO-anomalies, which have been detected in several Galactic globular clusters. The novelty and advantage of this study is that it is based on a homogeneous data set of hundreds of medium-resolution spectra of stars in eight Galactic globular clusters (M15, M22, M55, NGC288, NGC362, NGC5286, Palomar 12, and Terzan 7). Two of the clusters (Palomar 12 and Terzan 7) are believed to be former members of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy. The large homogeneous data set allows for a detailed differential study of the line strengths in the stellar spectra of the observed clusters. Our sample comprises stars in different evolutionary states, namely the main-sequence turn-off (MSTO) region, the subgiant branch (SGB) and the base of the red giant branch (RGB). We compare the relative CN and CH line strengths of stars in the same evolutionary states (with similar log g and T(eff)). The majority of the examined clusters show significant variations in their CN and CH abundances at the base of the RGB. We confirm the presence of a bimodal distribution in CN for the second parameter pair of the clusters (NGC288 and NGC362). The two probable former Sgr dSph clusters do not exhibit any CN-strong stars. Overall, our results suggest that the environment in which the clusters formed is responsible for the existence of CN-strong stars. We can confirm the known anticorrelation between CN and CH for most of the observed clusters. Although the signal of CN absorption is weaker for the hotter stars on the MSTO and SGB, we observed the same anticorrelation in these less evolved stars for the CN-bimodal clusters. Including structural parameters taken from the literature reveals that the existence of the CN-bifurcation seems to be independent of most other cluster characteristics. In particular, we do not confirm the correlation between cluster ellipticity and number of CN-strong stars. However, there may be a trend toward an increased percentage of CN-strong stars with increasing cluster tidal radius and total luminosity. We argue that our findings are consistent with pollution by intermediate AGB stars and/or fast rotating massive stars and two generations of star formation in luminous clusters with larger tidal radii at greater Galactocentric distances.