Microscopic diffusion and the calibration of globular cluster ages

We discuss current uncertainties in the predicted magnitudes of Turn-Off (TO) and Zero Age Horizontal Branch (ZAHB) stars, as produced by uncertainties in the efficiency of element sedimentation in low mass, metal poor stars. One finds that for ages of the order of 10 Gyr the uncertainties in sedimentation affect the calibration of the TO luminosity in terms of cluster ages less than 1 Gyr, this uncertainty becoming however larger for larger cluster ages. The difference in magnitude between TO and ZAHB stars, Delta V(TO-HB), appears less affected by sedimentation, since the variation in the TO luminosity is partially balanced by a similar variation of ZAHB luminosities. However additional uncertainties in the theoretical ZAHB luminosity are discussed, showing that a conservative estimate of uncertainties in the ZAHB V magnitude gives an error of +0.106/ - 0.047 and an uncertainty in the calibration of Delta V(TO-HB) in terms of the cluster age of the same order of the uncertainty in the calibration of TO magnitudes