On the circumstellar matter distribution around Herbig Ae/Be stars

The spectral energy distributions (SEDs) observed in Herbig Ae/Be stars are critically analyzed in connection with a spherically symmetric model for matter distribution around the central objects. The possibility that the dusty component is described by an MRN-DL mixture (see the work of Mathis, Rumpl, and Nordsiek, Draine and Lee) and is constituted by particles with mean sizes greater than those typical of the interstellar medium is considered. We find that while spherical models with interstellar-like dust are able to describe the observed SEDs in the visible, infrared, and radio region, they generally fail to fit the submillimeter fluxes. A better agreement is obtained if the absorption coefficient of the dust at far-infrared wavelengths follows a dependence that is shallower than that of the standard MRN-DL mixture. In this case, general agreement among the observed and the model-derived spectral types, distance, and visual extinction is found. Inconsistencies in submillimeter fluxes obtained by different authors are discussed in connection with the different telescopes used and the confusion of the sources. An indication is given for the density distribution around Herbig Ae/Be stars, which, if modeled with a power law, goes as the inverse of the radial distance in most cases.