Globular cluster luminosity functions and helium abundance differences across the galaxy

Preliminary results of a comparison of observed and theoretical giant branch luminosity functions for a large number of globular clusters are presented which may provide evidence for significant helium abundance differences among globular clusters. Bolometric luminosity functions constructed for 17 clusters on the basis of published photometry of the largest and most complete star samples were fit to theoretical luminosity functions appropriate to each cluster's Fe/H concentration ratio. Striking differences were observed from one cluster to another between the observed and predicted numbers of stars at the red giant tip. The ratio of the observed to the expected number of stars with bolometric magnitudes less than -3.0 was found to be correlated only with galactocentric distance for clusters within 10 kpc of the center, indicating that clusters at different distances do not all fit theoretical models with the same age, mass and chemical composition. It is argued that the only possible explanation for the correlation is that the helium abundance of clusters increases towards the galactic center, thus affecting the color magnitude diagrams. Further observations are required to estimate the actual helium abundances of these clusters, and to understand the globular clusters at galactocentric distances greater than 10 kpc.