Disks and halos in pre-main-sequence stars

Two different geometries have been proposed to explain the dust emission from pre-main-sequence 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, while 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. Even when its optical depth is much smaller than that of the disk, the halo can still have a significant effect on the disk temperature profile.