The Red Giant Branch Bump

We present a comparison between theoretical models and the observed magnitude difference between the horizontal branch and the red giant branch bump for a sample of 53 clusters. We find a general agreement, though some discrepancy is still present at the two extremes of the metallicity range of globular clusters. 1. The red giant branch bump The ‘bump’ is an intrinsic feature of the red giant branch (RGB) luminosity function (LF) of globular clusters (GC). It appears as a peak in the differential LF, and as a change in the slope of the cumulative LF. The bump originates when the H-burning shell crosses the chemical discontinuity left over by the convective envelope soon after the first dredge-up. So, a comparison between the theoretical and observed RGB bump location is a powerful tool for checking the capability of stellar models to finely predict the maximum inward extension of outer convection during the first dredge-up. In order to overcome the uncertainties related the GC distance scale, the location of the RGB bump is linked to the horizontal branch (HB), and the parameter ∆V Bump HB , (i.e., the difference in visual magnitude between the RGB bump and the HB stars within the RR Lyrae instability strip) is commonly used. Recently, an exhaustive comparison between theory and a large sample of GCs has been performed by Zoccali et al. (1999 Z99) and Ferraro et al. (1999). Here, we have doubled the Z99 sample (to 53 clusters), using the same HST snapshot database (Piotto et al. 2002). At variance with Z99, and in order to minimize the problems coming from the trasformation of the models from the theoretical to the observational plane, here we have worked directly in the HST F555W and F439W flight system. The analysis procedures are as in Z99. Figure 1 shows the comparison between the observed ∆F555W HB and the 1