The Chemical Evolution of Helium in Globular Clusters: Implications for   the Self-Pollution Scenario
  We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y from 0.28 to 0.40) compared to the primordial value. We assume that a previous generation of massive Asymptotic Giant Branch (AGB) stars have polluted the cluster. Two independent sets of AGB yields are used to follow the evolution of helium and CNO using a Salpeter initial mass function (IMF) and two top-heavy IMFs. In no case are we able to produce the postulated large Y ~ 0.35 without violating the observational constraint that the CNO content is nearly constant. 
Amanda Karakas, Yeshe Fenner, Alison Sills, Simon Campbell, John   Lattanzio