Disc clearing of young stellar objects: evidence for fast inside-out dispersal

The time-scale over which and the modality by which young stellar objects (YSOs) disperse their circumstellar discs dramatically influence the eventual formation and evolution of planetary systems. By means of extensive radiative transfer modelling, we have developed a new set of diagnostic diagrams in the infrared colour-colour plane (K - [24] versus K - [8]), to aid with the classification of the evolutionary stage of YSOs from photometric observations. Our diagrams allow the differentiation of sources with unevolved (primordial) discs from those evolving according to different clearing scenarios (e.g. homologous depletion versus inside-out dispersal), as well as from sources that have already lost their disc. Classification of over 1500 sources in 15 nearby star-forming regions reveals that approximately 39 per cent of the sources lie in the primordial disc region, whereas between 31 and 32 per cent disperse from the inside-out and up to 22 per cent of the sources have already lost their disc. Less than 2 per cent of the objects in our sample lie in the homogeneous draining regime. Time-scales for the transition phase are estimated to be typically a few 105 yr independent of stellar mass. Therefore, regardless of spectral type, we conclude that currently available infrared photometric surveys point to fast (of the order of 10 per cent of the global disc lifetime) inside-out clearing as the preferred mode of disc dispersal.