VERY SMALL DUST GRAINS IN THE CIRCUMSTELLAR ENVIRONMENT OF HERBIG AE BE STARS

We present calculations of the spectral energy distribution of Herbig Ae/Be stars surrounded by spherically symmetric, dusty envelopes where very small grains (VSGs) and polycyclic aromatic hydrocarbons (PAHs) are included. VSGs and PAHs radiate in the near and middle infrared, approximately from 2 to 20 mum. The resulting spectral energy distribution is quite flat in this range of wavelengths, depending mostly on the amount and size of the VSGs. Energetically, we find that VSGs and PAHs can contribute a significant fraction of the observed middle infrared luminosity, but not all, unless there is an extremely large amount of VSGs. Also, the model predicted near infrared colours indexes do not fit the observations significantly better than those of models where only large grains, in equilibrium with the local radiation field, are considered. Nevertheless, VSGs and PAHs do emit significantly more radiation in the mid-infrared than large grains. We suggest that VSGs and PAHs may be an important ingredient in models of Herbig Ae/Be stars as star+disk+envelope systems.