Red Giant Branch Bump Brightness and Number Counts in 72 Galactic Globular Clusters Observed with the Hubble Space Telescope

TL;DR

This study provides the most precise empirical measurements of the red giant branch bump brightness and counts across 72 globular clusters using HST data, revealing new dependencies and anomalies linked to multiple populations and helium abundance.

Contribution

It introduces a new method for measuring RGBB properties and offers the first comprehensive empirical calibration across a large sample of globular clusters.

Findings

RGBB brightness and counts depend on more than metallicity.

Anomalous RGBBs linked to multiple stellar populations.

Galactic bulge RGBB properties suggest higher helium abundance.

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 \textit{Hubble Space Telescope (HST)} globular cluster (GC) surveys to measure the brightness and star counts of the RGBB in 72 GCs. The brightness is measured to a precision better than 0.01 mag while the precision in number counts reaches 10%. The position of the main-sequence turnoff (MSTO) and the number of horizontal branch (HB) stars are used as comparisons where appropriate. Several independent scientific conclusions are newly possible with our parametrization 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 if the assumption of similar stellar physics for the bulge and Galactic GC system holds. The RGBB properties for the bulge are shown to differ from those of the Galactic GC system, 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, ${\Delta}$Y$\sim +$0.06 at the median metallicity.