A Survey of CN and CH Variations in Galactic Globular Clusters from SDSS Spectroscopy

TL;DR

This survey analyzes CN and CH band strengths in eight Galactic globular clusters using SDSS data, confirming CN bimodality in stars with metallicity above -1.7 and exploring correlations with metallicity and stellar evolution.

Contribution

It provides a homogeneous analysis of CN and CH variations across multiple clusters, extending previous findings with new evidence of bimodality and correlations.

Findings

CN bimodality confirmed in clusters with [Fe/H] > -1.7

Evidence of CN groups on subgiant and red giant branches

Correlation between metallicity and CN bandstrength slope

Abstract

We present a homogeneous survey of the CN and CH bandstrengths in eight Galactic globular clusters observed during the course of the Sloan Extension for Galactic Understanding and Exploration (SEGUE) sub-survey of the SDSS. We confirm the existence of a bimodal CN distribution among RGB stars in all of the clusters with metallicity greater than [Fe/H] = -1.7; the lowest metallicity cluster with an observed CN bimodality is M53, with [Fe/H] ~ -2.1. There is also some evidence for individual CN groups on the subgiant branches of M92, M2, and M13, and on the red giant branches of M92 and NGC 5053. Finally, we quantify the correlation between overall cluster metallicity and the slope of the CN bandstrength-luminosity plot as a means of further demonstrating the level of CN-enrichment in cluster giants. Our results agree well with previous studies reported in the literature.