ON THE ΔV bump HB PARAMETER IN GLOBULAR CLUSTERS

We present new empirical estimates of the ?V bump HB 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 & Ivans or by Carretta et al., we found that the observed ?V bump HB parameters are larger than predicted. In the metal-poor regime ([M/H] ?1.7, ?1.6?dex) 40% of GCs show discrepancies of 2? (0.40 mag) or more. Evolutionary models that account either for ?- 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 ?- and CNO-enhanced evolutionary models and observations in the Carretta et al. metallicity scale also indicates that observed ?V bump HB parameters, in the metal-rich regime ([M/H] ? 0), might be systematically smaller than predicted.