The Age‐dependent Luminosities of the Red Giant Branch Bump, Asymptotic Giant Branch Bump, and Horizontal Branch Red Clump

Abstract

Color-magnitude diagrams of globular clusters usually exhibit a prominent horizontal branch (HB) and may also show features such as the red giant branch (RGB) bump and the asymptotic giant branch (AGB) bump. Stellar evolution theory predicts that the luminosities of these features will depend on the metallicity and age of the cluster. We calculate theoretical lines of 2È12 Gyr constant age RGB bumps and AGB bumps in the diagram, which shows the brightness di †erence between the *V Bump HB-[Fe/H] bump and the HB as a function of metallicity. In order to test the predictions, we identify giant branch bumps in new Hubble Space T elescope color-magnitude diagrams for eight SMC clusters. First, we conclude that the SMC cluster bumps are RGB bumps. The data for clusters younger than D6 Gyr are in fair agreement with our predictions for the relative age-dependent luminosities of the HB and RGB bump. The data for clusters older than D6 Gyr demonstrate a less satisfactory agreement with *V Bump HB our calculations. We conclude that D6 Gyr is the lower bound to the age of clusters for which the Galactic globular cluster, age-independent calibration is valid. Application of the *V Bump HB-[Fe/H] diagram to stellar population studies is discussed.