1999MNRAS.303..265C

We present new evolutionary stellar models suitable for old Population I clusters, discussing both the consequences of the most recent improvements in the input physics and the effect of element diffusion within the stellar structures. Theoretical cluster isochrones are presented, covering the range of ages from 1 to 9 Gyr for the four selected choices of the metallicity Z= 0.007, 0.010, 0.015 and 0.020. Theoretical uncertainties on the efficiency of superadiabatic convection are discussed in some detail. Isochrone fitting to the CM diagrams of the two well-observed galactic clusters NGC 2420 and M67 indicates that a mixing-length parameter α = 1.9 appears adequate for reproducing the observed colour of cool giant stars. The problems in matching theoretical preditions to the observed slope of MS stars are discussed.