In this paper we estimate the initial He content in about 30 % of the Galactic globular clusters ( GGCs ) from new star counts we have performed on the recently published HST snapshot database of Colour Magnitude Diagrams ( Piotto et al .
2002 ) .
More specifically , we use the so-called R -parameter and estimate the He content from a theoretical calibration based on a recently updated set of stellar evolution models .
We performed an accurate statistical analysis in order to assess whether GGCs show a statistically significant spread in their initial He abundances , and whether there is a correlation with the cluster metallicity .
As in previous works on the subject , we do not find any significant dependence of the He abundance on the cluster metallicity ; this provides an important constraint for models of Galaxy formation and evolution .
Apart from GGCs with the bluest Horizontal Branch morphology , the observed spread in the individual helium abundances is statistically compatible with the individual errors .
This means that either there is no intrinsic abundance spread among the GGCs , or that this is masked by the errors .
In the latter case we have estimated a firm 1 \sigma upper limit of 0.019 to the possible intrinsic spread .
In case of the GGCs with the bluest Horizontal Branch morphology we detect a significant spread towards higher abundances inconsistent with the individual errors ; this can be fully explained by additional effects not accounted for in our theoretical calibrations , which do not affect the abundances estimated for the clusters with redder Horizontal Branch morphology .
In the hypothesis that the intrinsic dispersion on the individual He abundances is zero , taking into account the errors on the individual R -parameter estimates , as well as the uncertainties on the cluster metallicity scale and theoretical calibration , we have determined an initial He abundance mass fraction Y _ { GGC } = 0.250 \pm 0.006 .
This value is in perfect agreement with current estimates based on Cosmic Microwave Background radiation analyses and cosmological nucleosynthesis computations .