Irradiation Instability at the Inner Edges of Accretion Disks
  An instability can potentially operate in highly irradiated disks where the disk sharply transitions from being radially transparent to opaque (the 'transition region'). Such conditions may exist at the inner edges of transitional disks around T Tauri stars and accretion disks around AGNs. We derive the criterion for this instability, which we term the 'irradiation instability', or IRI. We also present the linear growth rate as a function of beta, the ratio between radiation force and gravity, and c_s, the sound speed of the disk, obtained using two methods: a semi-analytic analysis of the linearized equations and a numerical simulation using the GPU-accelerated hydrodynamical code PEnGUIn. In particular, we find that IRI occurs at beta~0.1 if the transition region extends as wide as ~0.05r, and at higher beta values if it is wider. This threshold value applies to c_s ranging from 3% of the Keplerian orbital speed to 5%, and becomes higher if c_s is lower. Furthermore, in the nonlinear evolution of the instability, disks with a large beta and small c_s exhibit 'clumping', extreme local surface density enhancements that can reach over ten times the initial disk surface density. 
Jeffrey Fung and Pawel Artymowicz
astro-ph.EP