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 \sim 0.1 if the transition region extends as wide as \sim 0.05 r , 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 .