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

Abstract

We have derived ages for 13 young (<30 Myr) star-forming regions and find\nthey are up to a factor two older than the ages typically adopted in the\nliterature. This result has wide-ranging implications, including that\ncircumstellar discs survive longer (~10-12 Myr) and that the average Class I\nlifetime is greater (~1 Myr) than currently believed.\n For each star-forming region we derived two ages from colour-magnitude\ndiagrams. First we fitted models of the evolution between the zero-age\nmain-sequence and terminal-age main-sequence to derive a homogeneous set of\nmain-sequence ages, distances and reddenings with statistically meaningful\nuncertainties. Our second age for each star-forming region was derived by\nfitting pre-main-sequence stars to new semi-empirical model isochrones. For the\nfirst time (for a set of clusters younger than 50 Myr) we find broad agreement\nbetween these two ages, and since these are derived from two distinct mass\nregimes that rely on different aspects of stellar physics, it gives us\nconfidence in the new age scale. This agreement is largely due to our adoption\nof empirical colour-Teff relations and bolometric corrections for\npre-main-sequence stars cooler than 4000 K.\n The revised ages for the star-forming regions in our sample are: ~2 Myr for\nNGC 6611 (Eagle Nebula; M 16), IC 5146 (Cocoon Nebula), NGC 6530 (Lagoon\nNebula; M 8), and NGC 2244 (Rosette Nebula); ~6 Myr for {\\sigma} Ori, Cep OB3b,\nand IC 348; ~10 Myr for {\\lambda} Ori (Collinder 69); ~11 Myr for NGC 2169; ~12\nMyr for NGC 2362; ~13 Myr for NGC 7160; ~14 Myr for {\\chi} Per (NGC 884); and\n~20 Myr for NGC 1960 (M 36).\n