The composition of the circumstellar dust around the Herbig Ae stars AB Aur and HD 163296

We have analysed the 2-200 mu m ISO spectra of two bright Herbig Ae stars, AB Aur and HD 163296, in order to derive the composition and mass over temp erature distribution of the circumstellar dust. We find that the two stars have a similar composition of the dust, however also with some important di fferences. We address the onset of the strong near-infrared emission betwee n 1-4 mu m. We show that this emission can be explained with thermal emissi on from metallic iron or dust species containing iron. The circumstellar du st surrounding both stars shows two distinct regimes in the mass over tempe rature distributions with temperatures of similar to 10(3) and similar to 1 0(2) K. Modelling of the ISO spectra further reveal a significant deviation of the derived grain sizes compared to the interstellar grain size distrib ution. A population of large (similar to 1 mm) grains is required to explai n the observed (sub-) millimetre fluxes of HD 163296. The presence of cryst alline silicates and the larger grain sizes in HD 163296 compared to AB Aur suggests that the former star is a more evolved system. Amorphous silicate s make out the bulk of the dust mass in both stars. It is likely that a sig nificant fraction of the dust is carbon-rich. We find strong evidence for a 20-30 mu m band which we tentatively attribute to FeO.