Probing the Helium Enrichment of the Galactic Bulge with the Red Giant Branch Bump David M. Nataf The Ohio State University Adviser: Andrew Gould Collaborators: M. Pinsonneault, J. Johnson, A. Udalski, E. Athanassoula

Bulge Star Formation History Matters ● Λ-CDM simulations fail at reproducing observed bulge distribution (Kormendy et al. 2010). ● Fundamental aspects of Galactic structure.

Galactic Bulge Stellar History: A Few Known Metallicity Constraints

Galactic Bulge Stellar History: Age Distribution a Matter of Controversy

Origin of the Elements

Helium Enrichment in Different Stellar Populations

Helium Can Probe Bulge Formation Environment

Enter the Red Giant Branch “Bump” (RGBB)

RGBB is Very Sensitive to Chemistry

The RGBB More Easily Detectable in Metal-Rich Systems ● “At higher metallicities the extension in luminosity of the bump is larger, and it is also shifter to lower luminosity, in more populated RGB regions, so it is easier to detect.” - Cassisi & Salaris (1997). ● “[The bump is harder to measure in] metal-poor clusters, where the bump moves toward brighter magnitudes and therefore less populated RGB regions.” - Zoccali et al. (1999). ● “identification of the RGB bump in metal-poor GCs is more difficult since it is brighter than the HB, where evolution along the RGB becomes faster and the stellar sample, consequently, becomes smaller.” - Di Cecco et al. (2010).

Total Absence of Results on the Bulge RGBB ● Perhaps because the RGBB is not well-known? It’s not even good enough for the textbooks.

Galactic Bulge Color Magnitude Diagram: No Prominent RGBB

How to Hide the RGBB: Enhanced Helium Enrichment ● Increased helium abundance would decrease the RGBB lifetime (and also increase the RC lifetime).

OGLE-III Galactic Bulge Photometric Survey, Udalski (2003, 2008)

The Challenge of Precisely Interpreting Galactic Bulge CMDs Too few stars (small radius)... or too much differential reddening (large radius).

Solution: Clump-Centric Color- Magnitude Diagrams (CCCMDs)

Probing Additional Post-MS Evolutionary Features ● Red Giant Branch Bump: ● ΔI = 0.74 ± 0.01, f = 20 ± 1%. ● Asymptotic Giant Branch Bump: ΔI ≈ -1.08, f ≈ 5.5%.

Comparing to Predictions – Metallicity Measurements as Inputs

Comparing to Predictions – Stellar Models

Comparing to Predictions – Empirical Globular Cluster Calibrations

Comparing to Predictions – Empirical Globular Cluster Calibrations Continued

Empirical Brightness Calibration – Success !

Empirical Number Counts Calibration – Questions, Questions and a 3-σ Detection

Caveat: GC Age-Helium-Metallicity Relation is a Matter of Ongoing Research. ● Globular Clusters have unique chemical self- enrichment histories that is thought to include heavy helium enhancement (Bedin et al. 2004, Carretta et al. 2010, etc). ● Nataf et al. (2011b) showed that the 47 Tuc RGBB has gradients in number counts and brightness with respect to distance from the cluster center.

Caveat: GC Age-Helium-Metallicity Relation is ill-defined Continued

Conclusions ● The RGBB is a completely independent probe of the Galactic bulge Age-Helium-Metallicity relation. ● Early analysis shows the bulge RGBB has deficient number counts relative to expectations from both models and globular clusters. ● Models do a fine job of matching data on the RGBB in Galactic globular clusters.