The clearing of discs around late-type T Tauri stars: constraints from the infrared two-colour plane

We have undertaken spectral energy distribution (SED) modelling of discs around low-mass T Tauri stars in order to delineate regions of the infrared two-colour plane (K − [8] versus K − [24]) that correspond to discs in different evolutionary stages. This provides a tool for classifying the nature of star–disc systems based on infrared photometry. In particular, we demonstrate the distinct loci followed by discs that undergo ‘uniform draining ’ (reduction in surface density by a spatially uniform factor) from those that clear from the inside-out. We draw attention to the absence of objects on this ‘draining locus ’ in those star-forming regions where the 24−µm sensitivity would permit their detection, as compared with the ∼20 objects in these regions with colours suggestive of inner holes. We thus conclude that discs predominantly clear from the inside-out. We also apply our classification of the infrared two-colour plane to stars of spectral types M3–M5 in the IC 348 cluster and show that some of its members (dubbed ‘weak excess ’ sources by Muzerolle et al.) that had previously been assumed to be in a state of clearing are instead likely to be optically thick discs in which the dust is well settled towards the mid-plane. Nevertheless, there are many discs in a state o