1995A&A...302..849N

We present models for the mid infrared emission of Herbig Ae/Be and T Tau stars. Special attention is given to the contribution from grains with temperature fluctuations consisting of PAHs, with their well known IR features, and very small graphite particles. We discuss in detail model calculated spectra between 3 and 15µm and the predicted brightness distribution at different wavelengths. The models are meant as a guide to observers. They have been obtained over a wide range of parameters for the star and its environment so that observed objects are bracketed by the set of models. In particular, we varied in the calculations the luminosity of the star and the optical depth and density distribution of the dust. We checked that our predicted emission, especially the strength of the 3.3 and 11.3µm PAH features, is in accord with present data. Our theoretical results on the features, their strength and intensity ratios, and the variation of the emission over the source may be used to derive some of the key parameters of pre-main-sequence stars. Furthermore, we calculate the percentage of stellar light that is absorbed by grains with temperature fluctions as a function of stellar type and for various PAH cross sections; values vary between 10 and 35%. We also study the influence of presently poorly determined PAH properties, like their size and optical or UV cross section, on the spectrum. We argue that PAH features are not observed in T Tau stars, although the models predict them, because they may be swamped by the continuum emission of the star or disk.