THE STRUCTURE AND EMISSION OF ACCRETION DISKS IRRADIATED BY INFALLINGENVELOPES

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