  Using a chemical evolution model we investigate the intriguing suggestion
that there are populations of stars in some globular clusters (e.g. NGC 2808,
omega Centauri) with enhanced levels of helium (Y from about 0.28 to 0.40)
compared to the majority of the population that presumably have a primordial
helium abundance. We assume that a previous generation of massive
low-metallicity Asymptotic Giant Branch (AGB) stars has polluted the cluster
gas via a slow stellar wind. We use two independent sets of AGB yields computed
from detailed models to follow the evolution of helium, carbon, nitrogen and
oxygen in the cluster gas using a Salpeter initial mass function (IMF) and a
number of top-heavy IMFs. In no case were we able to fit the observational
constraints, Y > 0.30 and C+N+O approximately constant. Depending on the shape
of the IMF and the yields, we either obtained Y approximately greater than 0.30
and large increases in C+N+O or Y < 0.30 and C+N+O approximately constant.
These results suggest that either AGB stars alone are not responsible for the
large helium enrichment or that any dredge-up from this generation of stars was
less than predicted by standard models.
