2000ApJ...538..289Z

We estimate the empirical R-parameter in 26 Galactic globular clusters covering a wide metallicity range, imaged by Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The improved spatial resolution permits a large fraction of evolved stars to be measured and permits accurate assessment of radial population gradients and completeness corrections. In order to evaluate both the He abundance and the He-to-metal enrichment ratio, we construct a large set of evolutionary models by adopting similar metallicities and different He contents. We find an absolute He abundance that is lower than that estimated from spectroscopic measurements in H II regions and from primordial nucleosynthesis models. This discrepancy could be removed by adopting a ¹²C(α,γ)¹⁶O nuclear cross section about a factor of 2 smaller than the canonical value, although different assumptions for mixing processes also can introduce systematic effects.

The trend in the R-parameter toward solar metallicity is consistent with an upper limit to the He-to-metal enrichment ratio of the order of 2.5. Detailed calculations of central He burning times as a function of the horizontal-branch (HB) morphology suggest that He lifetimes for hot HB stars are on average ~20% longer than for RR Lyrae and red HB stars. Therefore, the increase in the empirical R-values of metal-poor clusters characterized by blue HB morphologies is due to an increase in the HB lifetime and not due to an increase in the He abundance.