The red giant branch luminosity function bump.

We present observational estimates of the magnitude difference between the luminosity function red giant branch bump and the horizontal branch (Delta F555WbumpHB), and of star counts in the bump region (Rbump), for a sample of 54 Galactic globular clusters observed by the HST. The large sample of stars resolved in each cluster, and the high photometric accuracy of the data allowed us to detect the bump also in a number of metal poor clusters. To reduce the photometric uncertainties, empirical values are compared with theoretical predictions obtained from a set of updated canonical stellar evolution models which have been transformed directly into the HST flight system. We found an overall qualitative agreement between theory and observations. Quantitative estimates of the confidence level are hampered by current uncertainties on the globular cluster metallicity scale, and by the strong dependence of Delta F555WbumpHB on the cluster metallicity. In case of the Rbump parameter, which is only weakly affected by the metallicity, we find a very good quantitative agreement between theoretical canonical models and observations. For our full cluster sample the average difference between predicted and observed Rbump values is practically negligible, and ranges from -0.002 to -0.028, depending on the employed metallicity scale. The observed dispersion around these values is entirely consistent with the observational errors on Rbump. As a comparison, the value of Rbump predicted by theory in case of spurious bump detections due to Poisson noise in the stellar counts would be ~ 0.10 smaller than the observed ones. We have also tested the influence on the predicted Delta F555WbumpHB and Rbump values of an He-enriched component in the cluster stellar population, as recently suggested by D'Antona et al. (\cite{d02}). We find that, under reasonable assumptions concerning the size of this He-enriched population and the degree of enrichment, the predicted Delta F555WbumpHB and Rbump values are only marginally affected. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555, and on observations retrieved with the ESO ST-ECF Archive.