Disks and Halos in Pre-Main-Sequence Stars

Two different geometries have been proposed to explain the dust emission from pre-mainsequence stars: flared disks and “classical” geometrically-thin optically-thick disks imbedded in optically thin halos. We show that only imaging observations can differentiate these two morphologies because flux measurements can never distinguish between them. A model constructed with one geometry implies an equivalent model with the other that produces the identical flux at every wavelength. When a flared disk is imbedded in a halo, the halo dominates the system IR flux whenever τhalo > 14H/R, where τhalo is the optical depth across the halo at visual wavelengths and R and H are, respectively, the disk radius and height at its outer edge. Even when its optical depth is much smaller, the halo can still have a significant effect on the disk temperature profile.