The initial helium content of Galactic globular cluster stars from the R-parameter:: Comparison with the cosmic microwave background constraint

Recent precise determinations of the primordial He abundance (Y-p) from cosmic microwave background (CMB) analyses and cosmological nucleosynthesis computations provide Y-p=0.248+/-0.001. On the other hand, recent works on the initial He abundance of Galactic globular cluster (GGC) stars, making use of the R-parameter as an He indicator, have consistently obtained Y(GGC)similar to0.20. In light of this serious discrepancy, which casts doubt on the adequacy of low-mass He-burning stellar models, we have rederived the initial He abundance for stars in two large samples of GGCs by employing theoretical models computed using new and more accurate determinations of the equation of state for the stellar matter and of the uncertain C-12(alpha,gamma)O-16 reaction rate. Our models include semiconvection during the central convective He-burning phase, while the breathing pulses are suppressed, in agreement with the observational constraints coming from the measurements of the R-2-parameter in a sample of clusters. By taking into account the observational errors on the individual R-parameter values, as well as uncertainties in the GGC [Fe/H] scale, treatment of convection, and C-12(alpha,gamma)O-16 reaction rate, we have obtained a mean Y-GGC=0.243+/-0.006 and Y-GGC=0.244+/-0.006 for the two studied GGC samples. These estimates are now fully consistent with Y-p obtained from CMB studies. Moreover, the trend of the individual He abundances with respect to [Fe/H] is consistent with no appreciable He enrichment along the GGC metallicity range.