Abundance sensitive parameters for red giants in globular clusters

Photoelectric VIKC photometry yields structural parameters for the giant branch in nine clusters. Three features are studied spectroscopically in the 5000–6200 Å region. The TiO bands are closely correlated with V−I colour and do not provide an independent abundance criterion. The Mg b lines depend on abundance but lose sensitivity at high metal content, apparently because of the strengthening of the overlapping MgH bands. The Na D lines are adversely affected by interstellar absorption and atmospheric emission. They show a wide scatter at a given colour in 47 Tuc, similar to that found among hotter stars, as well as a strong positive correlation with metal abundance. These and other parameters obtained from photometry and low-resolution spectroscopy order the clusters in a similar way which is at variance with the results of abundance analyses based on high-dispersion spectroscopy. This does not seem explicable in terms of differences in helium abundance. Several stars lying above the giant branch also show TiO bands, including the RV Tauri variable V17 and the W Virginis star V21 in NGC 6626. They are the first variables of their type in a globular cluster to show TiO bands.