On the Helium Content of Galactic Globular Clusters via theR‐Parameter

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 12 C(α,γ) 16 O 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.