SUBMILLIMETER OBSERVATIONS OF HERBIG AE/BE SYSTEMS

Pre-main-sequence, intermediate-mass stars have been observed in continuum emission between 0.35 and 1.3 mm in an effort to study the geometrical distribution of circumstellar dust. Flux measurements were made for a total of 13 Herbig Ae/Be systems,and the use of eight filter bandpasses has now essentially filled-in much of the submm portion of the spectral energy distributions of these objects for the first time. Eight of the sources are classified as Group I (possibly disc-dominated) systems by Hillenbrand et al. The remaining five sources belong to Group II and are characterized by substantial far-IR and mm-wave emission from extended dusty envelopes. The new measurements of 1.3-mm flux provide rough estimates of the total mass of dust and gas in each source. It is shown that flat, disc-like distributions of material would be consistent with these masses for the Group I sources. However, it is also found that, with the exception of the Herbig Ae/Be system HD 163296, the same masses distributed within spherical envelopes matched to the 20-arcsec-FWHM beam would lead to V-band extinctions that are consistent with observed values towards these stars. To decide between the two geometries, the observed Group I submm spectral energy distributions (SEDs) have been compared with SEDs computed using a standard disc model [T(r)proportional to r(-3/4)]. Two of the Group I sources - AB Aur and MWC 147 - can be modelled by assuming that a disc either dominates, or is entirely responsible for, all excess emission. Other geometrical distributions are not ruled out for these sources, but it is claimed that a pure disc model is not inconsistent with available measurements. Four of the Group I sources - MWC 137, Lk H alpha 215, HD 163296 and MWC 1080 have measured 0.35- or 0.45-mm flux densities which are significantly in excess of those which can be generated even by standard discs which are entirely optically thick at all wavelengths: long-wavelength emission from these sources is possibly dominated by flared discs and/or extended dust components. Finally, it is speculated that possible optically thin mm/submm disc emission in HD 163296 may provide indirect evidence of the presence of large grains.