The Structure and Emission of Accretion Disks Irradiated by Infalling Envelopes

We calculate the emission from steady viscous disks heated by radiation from an opaque infalling protostellar envelope. For typical envelope parameters used to explain the spectral energy distributions of protostellar sources, we find that the envelope heating raises the outer disk temperature dramatically. The resulting temperature distribution in the disk is a complicated function of both radial distance and vertical height above the disk midplane. We show that the visibility flux at λ = 0.87 mm and the spectral energy distribution from submillimeter to radio wavelengths of the flat-spectrum T Tauri star HL Tau can be explained by emission from an accretion disk irradiated by its infalling envelope, whereas thermal emission from an infalling envelope or radiation from a steady viscous accretion disk cannot explain the observations. Our results suggest that the radiation fields of collapsing protostellar envelopes may strongly affect the structure of pre-main-sequence accretion disks.