ON THE ΔV bumpHB PARAMETER IN GLOBULAR CLUSTERS

We present new empirical estimates of the ΔV bumpHB parameter for 15 Galactic globular clusters (GGCs) using accurate and homogeneous ground-based optical data. Together with similar evaluations available in the literature, we ended up with a sample of 62 GGCs covering a very broad range in metal content (-2.16 dex ≤ [M/H] ≤ -0.58 dex). Adopting the homogeneous metallicity scale provided either by Kraft and Ivans or by Carretta et al., we found that the observed ΔV bumpHB parameters are larger than predicted. In the metal-poor regime ([M/H] ∼< -1.7, -1.6 dex) 40% of GCs show discrepancies of 2σ (∼0.40 mag) or more. Evolutionary models that account either for α- and CNO-enhancement or for helium enhancement do not alleviate the discrepancy between theory and observations. The outcome is the same if we use the new solar heavy-element mixture. The comparison between α- and CNO-enhanced evolutionary models and observations in the Carretta et al. metallicity scale also indicates that observed ΔV bumpHB parameters, in the metal-rich regime ([M/H] ≥ 0), might be systematically smaller than predicted.