Empirical isochrones and relative ages for young stars, and the radiative-convective gap

We have selected pre-main-sequence (PMS) stars in 12 groups of notional ages ranging from 1 to 35 Myr, using heterogeneous membership criteria. Using these members we have constructed empirical isochrones in V, V - I colour-magnitude diagrams. This allows us to identify clearly the gap between the radiative main sequence and the convective PMS (the R-C gap). We follow the evolution of this gap with age and show that it can be a useful age indicator for groups less than similar or equal to 15 Myr old. We also observe a reduction in absolute spreads about the sequences with age. Finally, the empirical isochrones allow us to place the groups in order of age, independently of theory. The youngest groups can be collated into three sets of similar ages. The youngest set is the ONC, NGC 6530 and IC 5146 (nominally 1 Myr); next Cep OB3b, NGC 2362, lambda Ori and NGC 2264 (nominally 3 Myr); and finally sigma Ori and IC 348 (nominally 4-5 Myr). This suggests Cep OB3b is younger than previously thought, and IC 348 older. For IC 348 the stellar rotation rate distribution and fraction of stars with discs imply a younger age than we derive. We suggest this is because of the absence of O-stars in this cluster, whose winds and/or ionizing radiation may be an important factor in the removal of discs in other clusters.