COMPARISON BETWEEN PREDICTED AND EMPIRICAL ∆ V BumpHB IN GALACTIC GLOBULAR CLUSTERS

We present observational estimates of ∆V Bump HB in a sample of 28 Galactic Globular Clusters (GGCs) observed by HST. The photometric accuracy and the sizable number of stars measured in each cluster allowed us to single out the RGB Bump both in metal-poor and in metal-rich GGCs. Empirical values are compared with homogeneous theoretical predictions which 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 (1997) as extended by Cohen et al. (1999) is adopted. Interestingly enough, we also found that both theoretical and observed values show a change in the slope of the ∆V Bump HB-[M/H] relation toward higher metal contents. The RGB Bump is an intrinsic feature of the RGB Lu-minosity Function (LF) of GGCs. It appears as a peak in the differential LF or as a change in the slope of the cumulative LF. The presence of the Bump is due to the fact that during the RGB evolution the H-burning shell crosses the chemical discontinuity left over by the convective envelope soon after the first dredge-up phase. Since its first detection in 47 Tuc (King, Da Costa & Demarque 1985), it became the crossroad of several theoretical and observational investigations (Alves & Sarajedini 1999). The detection of the RGB Bump was mainly hampered by the size of the available samples of RGB stars. This is particularly true for the most metal-poor clusters, where the Bump moves toward brighter magnitudes and therefore less populated RGB regions. Only recently was this feature firmly detected in a large set of both Galactic (FP; Brocato et al. 1996) and extragalactic stellar systems (Desidera 1999). The analysis by FP showed that the ∆V Bump HB 5 values predicted by theory were 0.4 mag brighter than the empirical estimates in a sample of 11 GGCs. This observable, which does not depend on the distance modulus, on the reddening, or on the calibration of photometric data, is a key parameter for assessing both the accuracy and the plausibility of the physical assumptions adopted for constructing the evolutionary models. As a consequence the discrepancy found by FP needs to be understood. In order to explain this mismatch between theory and observations Alongi …