DUST EMISSION FROM HERBIG Ae/Be STARS: EVIDENCE FOR DISKS AND ENVELOPES

Infrared and millimeter-wave emission from Herbig Ae/Be stars has produced conflicting conclusions regarding the dust geometry in these objects. We show that the compact dimensions of the millimeter-wave–emitting regions are a decisive indication for disks. But a disk cannot explain the spectral energy distribution unless it is embedded in an extended envelope that (1) dominates the IR emission and (2) provides additional disk heating on top of the direct stellar radiation. Detailed radiative transfer calculations based on the simplest model for envelopeembedded disks successfully fit the data from UV to millimeter wavelengths and show that the disks have central holes. This model also resolves naturally some puzzling results of IR imaging. Subject headings: accretion, accretion disks — circumstellar matter — dust, extinction — stars: pre–main-sequence