Comparison between Predicted and Empirical ΔV^bump_HB in Galactic Globular Clusters

Abstract

We present observational estimates of ΔV^bump_HB in a sample of 28 galactic globular clusters (GGCs) observed by the Hubble Space Telescope. The photometric accuracy and the sizable number of stars measured in each cluster allowed us to single out the red giant branch bump both in metal-poor and in metal-rich GGCs. Empirical values are compared with homogeneous theoretical predictions that account for both H- and He-burning phases over a wide range of metal abundances (0.0001<Z<0.02). We found that, within current observational uncertainties on both iron and α-element abundances, theory and observations are in very good agreement, provided that the metallicity scale by Carretta & Gratton as extended by Cohen et al. is adopted. Interestingly enough, we also found that both theoretical and observed values show a change in the slope of the ΔV^bump_HB-[M/H] relation toward higher metal contents.

The parameter ΔV^bump_HB is commonly defined as the difference in magnitude between the red giant branch bump and the horizontal-branch stars located within the RR Lyrae instability strip (for a detailed discussion, see Fusi Pecci et al. 1990 and Cassisi & Salaris 1997).