On the thermal stability of irradiation-dominated pre-main-sequence disks

The dusty disks of many young stars are probably heated mostly by absorptio n of light from the central star. This stellar irradiation can control the vertical structure of the disk, particularly in the outer regions. Because the irradiation heating is sensitive to the disk structure-the disk vertica l thickness and the tilt of the disk photosphere relative to the star-the p ossibility of an unstable feedback is present. To study this problem, we pr esent calculations of the evolution of perturbations in vertically isotherm al disks. We find that such disks are generally stable. In outer disk regio ns of T Tauri stars, linear analysis indicates that the radiative cooling t ime is so short that temperature perturbations will be damped faster than t he disk structure can respond. Using our results for steady "alpha" viscosi ty disks, we estimate that this is true for distances larger than 2 AU ((M) over dot/10(-8) M. yr(-1))(7/9)(alpha/0.01)(-7/9) for typical T Tauri star s. Inside this radius, if the disk surface tilt ("flaring") is still signif icant, numerical finite-amplitude calculations show that temperature pertur bations will travel inward as they damp. We find that disk self-shadowing h as a small effect on the results because the perturbation is damped on a ti mescale shorter than the time in which the shadowed disk region can respond . Our results help justify steady, smooth treatments of the effects of irra diation on the disks of young stellar objects.