Helium abundance in globular clusters: the R-method.

A new estimate of the helium abundance, Y, in globular cluster stars has been obtained through the application of the R-method of Iben (1968) to a sample of galactic globular clusters. A new calibration for the (R-Y) relation has been derived. The influence on the calibration of some theoretical uncertainties and of phenomena like mass loss have been analyzed. The procedure, applied to a sample of 15 clusters whose data reach a level of completeness and photometric accuracy enough to give statistically significant results, led to a mean value for Y equal to 0.23 + or - 0.02. A constant Y with varying metal content seems consistent with the data. The distribution of the number ratios of asymptotic and giant branch stars suggests the full development of the semiconvective zone for the stars of all the considered clusters.