Fitting the young main-sequence: distances, ages and age spreads

We use several main-sequence models to derive distances (and extinctions), with statistically meaningful uncertainties for 11 star-forming regions and young clusters. The model dependency is shown to be small, allowing us to adopt the distances derived using one model. Using these distances, we have revised the age order for some of the clusters of Mayne et al. The new nominal ages are: ~2 Myr for NGC 6530 and the ONC, ~3 Myr for λ Orionis, NGC 2264 and σ Orionis, ~4-5 Myr for NGC 2362, ~13 Myr for h and χ Per, ~20 Myr for NGC 1960 and ~40 Myr for NGC 2547. In cases of significantly variable extinction, we have derived individual extinctions using a revised Q-method. These new data show that the largest remaining uncertainty in deriving an age ordering (and necessarily ages) is metallicity. We also discuss the use of a feature we term the radiative-convective gap overlap to provide a diagnostic of isochronal age spreads or varying accretion histories within a given star formation region. Finally, recent derivations of the distance to the ONC lie in two groups. Our new more precise distance of 391⁺¹²_-9 pc allows us to decisively reject the further distance; we adopt 400 pc as a convenient value.