Radiation from the central star excites particles in the accretion disk. The irradiation of the disk gives rise to a stability length scale known as the gravitational radius ( r g ). Outside of the gravitational radius, particles can become sufficiently excited to escape the gravity of the disk, and evaporate. After 10 – 10 years, the viscous accretion rates fall below the photoevaporation rates at r g . A gap then opens around r g , the inner disk drains onto the central star, or spreads to r g and evaporates. An inner hole extending to r g is produced. Once an inner hole forms, the outer disk is very rapidly cleared.