An analysis of G-band strengths in NGC 6397 and M55 red giants

A spectroscopic index which measures the strength of the G band has been determined for a number of red giants in the very metal-poor globular clusters NGC 6397 and M55. An analysis of these indices indicates a clear pattern of decreasing carbon abundances as a function of increasing evolutionary state in NGC 6397. The asymptotic giant branch (AGB) stars in NGC 6397 appear to have anomalous weak G bands compared to red giant branch stars of similar color. The weak G-band effect appears to be largely the results of a mild carbon depletion typical of evolved metal-poor stars which has been exaggerated by the lower surface gravities of AGB stars. Nitrogen abundances have also determined for the five brightest stars in NGC 6397.