David Nataf
Ohio State University
& NOAO South Visiting Astronomer
 

"The Red Giant Branch Bump as a Tracer of Helium Enrichment in the Galactic Bulge"
 

I use VI color-magnitude diagrams from the 96 deg² OGLE-III Galactic Bulge photometric survey to measure the Galactic bulge Red Giant Branch Bump (RGBB), the most metal-rich RGBB ever detected. The location of the RGBB (~0.7 mag fainter than the red clump) is as expected from empirical calibrations and theoretical expectations, but the number density of RGBB stars (~13% that of the red clump)  is down by a factor of ~2.0. Together, these two observations are most consistent with an enhanced helium enrichment for the Galactic bulge, such that Y~0.35 rather than the commonly assumed 0.27. I also show that stellar evolution models do an excellent job of predicting RGBB star counts in Galactic globular clusters.