The lifetime of protoplanetary discs: Observations and Theory
  The time-scale over which and modality by which young stellar objects (YSOs)
disperse their circumstellar discs dramatically influences the eventual
formation and evolution of planetary systems. By means of extensive radiative
transfer (RT) modelling, we have developed a new set of diagnostic diagrams in
the infrared colour-colour plane (K-[24] vs. K-[8]), to aid with the
classiffication of the evolutionary stage of YSOs from photometric
observations. Our diagrams allow the differentiation of sources with un-evolved
(primordial) discs from those evolving according to different clearing
scenarios (e.g. homologous depletion vs. 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% of the sources
lie in the primordial disc region, whereas between 31% and 32% disperse from
the inside-out and up to 22% of the sources have already lost their disc. Less
than 2% 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 10^5
years independent of stellar mass. Therefore, regardless of spectral type, we
conclude that currently available infrared photometric surveys point to fast
(of order 10% of the global disc lifetime) inside-out clearing as the preferred
mode of disc dispersal.
