Red giant branch bump brightness and number counts in 72 galactic globular clusters observed with the Hubble space telescope

Abstract

We present the broadest and most precise empirical investigation of red giant branch bump (RGBB) brightness and number counts ever conducted. We implement a new method and use data from two Hubble Space Telescope globular cluster (GC) surveys to measure the brightness and star counts of the RGBB in 72 GCs. The median measurement precision is 0.018 mag in the brightness and 31% in the number counts, respectively, reaching peak precision values of 0.005 mag and 10%. The position of the main-sequence turnoff and the number of horizontal branch stars are used as comparisons where appropriate. Several independent scientific conclusions are newly possible with our parameterization of the RGBB. Both brightness and number counts are shown to have second parameters in addition to their strong dependence on metallicity. The RGBBs are found to be anomalous in the GCs NGC 2808, 5286, 6388, and 6441, likely due to the presence of multiple populations. Finally, we use our empirical calibration to predict the properties of the Galactic bulge RGBB. The updated RGBB properties for the bulge are shown to differ from the GC-calibrated prediction, with the former having lower number counts, a lower brightness dispersion, and a brighter peak luminosity than would be expected from the latter. This discrepancy is well explained by the Galactic bulge having a higher helium abundance than expected from GCs, ΔY ∼ +0.06 at the median metallicity.