Pre-main-sequence isochrones - II. Revising star and planet formation time-scales

We have derived ages for 13 young (< 30 Myr) star-forming regions and find that they are up to a factor of 2 older than the ages typically adopted in the literature. This result has wide-ranging implications, including that circumstellar discs survive longer (similar or equal to 1012 Myr) and that the average Class I lifetime is greater (similar or equal to 1 Myr) than currently believed. For each star-forming region, we derived two ages from colour-magnitude diagrams. First, we fitted models of the evolution between the zero-age main sequence and terminal-age main sequence to derive a homogeneous set of main-sequence ages, distances and reddenings with statistically meaningful uncertainties. Our second age for each star-forming region was derived by fitting pre-main-sequence stars to new semi-empirical model isochrones. For the first time (for a set of clusters younger than 50 Myr), we find broad agreement between these two ages, and since these are derived from two distinct mass regimes that rely on different aspects of stellar physics, it gives us confidence in the new age scale. This agreement is largely due to our adoption of empirical colour-T-eff relations and bolometric corrections for pre-main-sequence stars cooler than 4000 K. The revised ages for the star-forming regions in our sample are: similar to 2 Myr for NGC 6611 (Eagle Nebula; M 16), IC 5146 (Cocoon Nebula), NGC 6530 (Lagoon Nebula; M 8) and NGC 2244 (Rosette Nebula); similar to 6 Myr for Sigma Ori, Cep OB3b and IC 348; similar or equal to 10 Myr for lambda Ori (Collinder 69); similar or equal to 11 Myr for NGC 2169; similar or equal to 12 Myr for NGC 2362; similar or equal to 13 Myr for NGC 7160; similar or equal to 14 Myr for chi Per (NGC 884); and similar or equal to 20 Myr for NGC 1960 (M 36).