The destruction of circumstellar discs

We present the results of Nbody simulations of the Orion Nebula Cluster which keep track of the destruction of discs by star-disc collisions and photoevaporation. We find the former process to be negligible, but that photoionisation is highly significant, although planet forming discs should still be able to survive in the outer reaches of the cluster. We find that orbital dynamics cannot solve the proplyd frequency problem. We also present the results of a new 'ultraviolet switch' model for the dispersal of discs around T Tauri stars, which combines photoevaporation by the central star with viscous evolution. This model successfully reproduces the observed 'two timescale' nature of disc dispersal and promotes the rapid (similar to 10(5) year) demise of the disc within 5 - 10 AU. Observational tests and some implications for planet formation are discussed.