OGLE-III Detection of the Anomalous Galactic Bulge Red Giant Branch Bump: Evidence of Enhanced Helium Enrichment

Abstract

We measure the red giant branch bump (RGBB) of the Galactic bulge, the most metal-rich RGBB ever detected. The RGBB luminosity functions peaks at the expected brightness, but its number density is very low relative to Galactic globular cluster calibrations, implying the Galactic bulge has a higher helium enrichment parameter ΔY/ΔZ $\ge$ 4.0 for Y~0.35 rather than the standard 2.0 with Y=0.27. The RGBB is (0.71 +/- 0.02) mag fainter than the red clump (RC) in I toward the densest stellar regions imaged by the OGLE-III Galactic bulge photometric survey, (|l| $\le$ 4, 2 <~ |b| $\le$ 4). The number density of RGBB stars is (12.7 +/- 2.0)% that of RC stars. The brightness dispersion of the RGBB is significantly lower than that of the RC, a result that is difficult to explain as the luminosity of the RGBB is known to significantly vary with metallicity. Sightlines toward the Galactic bulge that have two RCs have two RGBBs with similar properties to one another, an expected outcome if the Milky Way's bulge is X-shaped. We also find preliminary evidence of the Galactic bulge asymptotic giant branch bump, at a brightness of ~1.1 mag brighter than the RC in I and with a number density ~1.5% that of the RC. Accounting for the RGBB has a small effect on the best-fit parameters of the RC, shifting its best-fit peak brightness and reducing its brightness dispersion by ~0.015 mag each.