A CRITICAL INVESTIGATION ON THE DISCREPANCY BETWEEN THE OBSERVATIONALAND THE THEORETICAL RED GIANT LUMINOSITY FUNCTION BUMP

We present theoretical evaluations of the red giant branch (RGB) lumin osity function 'bump' and the zero-age horizontal branch (ZAHB) lumino sity 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 v alue, age, mass loss, bolometric corrections, opacities and equation o f state adopted in the evolutionary models is also discussed. These ne w prescriptions have been taken into account to cast light on a longst anding 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 accura te 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 comp ared with theoretical values. It is shown that there is no significant disagreement between the observations and the canonical stellar model s. The possible applications of this result are also briefly discussed .