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

Color-magnitude diagrams of globular clusters often exhibit a prominenthorizontal branch (HB) and may also show features such as the red giant branch(RGB) bump and the asymptotic giant branch (AGB) bump. Stellar evolution theorypredicts that the luminosities of these features will depend on the metallicityand age of the cluster. We calculate theoretical lines of 2 to 12 Gyr constantage RGB-bumps and AGB-bumps in the V(HB-Bump)--[Fe/H] diagram, which shows thebrightness difference between the bump and the HB as a function of metallicity.In order to test the predictions, we identify giant branch bumps in new HubbleSpace Telescope color-magnitude diagrams for 8 SMC clusters. First, we concludethat the SMC cluster bumps are RGB-bumps. The data for clusters younger than ~6Gyr are in fair agreement the relative age dependent luminosities of the HB andRGB-bump. The V(HB-Bump)--[Fe/H] data for clusters older then ~6 Gyrdemonstrate a less satisfactory agreement with our calculations. We concludethat ~6 Gyr is a lower bound to the age of clusters for which the Galacticglobular cluster, age independent V(HB-Bump)--[Fe/H] calibration is valid.Application of the V(HB-bump)--[Fe/H] diagram to stellar population studies isdiscussed.Comment: Accepted for publication in the Astrophysical Journal, 30 pages, Latex aaspp4.sty, including 7 postscript figure