Destruction of Stellar Disks by Photoevaporation
  Photoevaporation may provide an explanation for the short lifetimes of disks around young stars. With the exception of neutral oxygen lines, the observed low-velocity forbidden line emission from T Tauri stars can be reproduced by photoevaporating models. The natural formation of a gap in the disk at several AU due to photoevaporation and viscous spreading provides a possible halting mechanism for migrating planets and an explanation for the abundance of observed planets at these radii. 
D. Johnstone, I. Matsuyama, I.G. McCarthy, A.S. Font