A critical investigation on the discrepancy between the observational and the theoretical red giant luminosity function 'bump'

We present theoretical evaluations of the red giant branch (RGB) luminosity function 'bump' and the zero-age horizontal branch (ZAHB) luminosity covering the range of metallicities typical of Galactic globular clusters. The variation of the theoretical RGB bump and ZAHB levels as a result of the metallicity, original helium content, mixing length value, age, mass loss, bolometric corrections, opacities and equation of state adopted in the evolutionary models is also discussed. These new prescriptions have been taken into account to cast light on a longstanding astrophysical problem connected with the RGB evolutionary phase, namely the discrepancy between the observational and the theoretical luminosity of the RGB bump. A sample of globular clusters with accurate evaluations of the bump luminosity and spectroscopically determined metallicities has been selected. The ZAHB luminosity at the RR Lyrae instability strip has been accurately evaluated, and the observational luminosity difference between the RGB bump and the ZAHB has been compared with theoretical values. It is shown that there is no significant disagreement between the observations and the canonical stellar models. The possible applications of this result are also briefly discussed.