On the ΔV bump HB Parameter in Globular Clusters

We present new empirical estimates of the ΔV 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 ΔV 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 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 bump HB parameters, in the metal-rich regime ([M/H] ≥ 0), might be systematically smaller than predicted.