The Uncertainties in the Age of Globular Clusters from Their Helium Abundance and Mass Loss

Abstract

The Uncertainties in the Age of Globular Clusters from Their Helium Abundance and Mass Loss X. Shi The University of Chicago, Chicago, IL 60637-1433 Abstract The age of globular clusters inferred from observations depends sensitively on assumptions such as the initial helium abundance and the mass loss rate. A high helium abundance (e.g., Y zzO.28), as well as an inclusion of helium diffusion and oxygen-enhancement in stellar models, can lower the current age estimate for metal-poor globular clusters from 14f1.5 Gyr to about llfl Gyr, significantly relaxing the constraints on the Hubble constant, allowing values as high as GOkm/sec/Mpc for a universe with the critical density and SOkm/sec/Mpc for a baryon-only universe. The uncertainties of a high helium abundance and an instability strip induced mass loss near the turn-off in globular clusters are discussed. Ages lower than 10 Gyr are not possible even with the operation of both of these mechanisms unless the initial helium abundance in globular clusters is > 0.28, which can hardly be accomodated by indirect inferences of helium abundances in globular clusters.