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

Abstract. We have analysed the 2–200 µm ISO spectra of two bright Herbig Ae stars, AB Aur and HD 163296, in order to derive the composition and mass over temperature distribution of the circumstellar dust. We find that the two stars have a similar composition of the dust, however also with some important differences. We address the onset of the strong near-infrared emission between 1–4 µm. We show that this emission can be explained with thermal emission from metallic iron or dust species containing iron. The circumstellar dust surrounding both stars shows two distinct regimes in the mass over temperature distributions with temperatures of ∼ 10 3 and ∼ 10 2 K. Modelling of the ISO spectra further reveal a significant deviation of the derived grain sizes compared to the interstellar grain size distribution. A population of large ( ∼ 1 mm) grains is required to explain the observed (sub-) millimetre fluxes of HD 163296. The presence of crystalline silicates and the larger grain sizes in HD 163296 compared to AB Aur suggests that the former star is a more evolved system. Amorphous silicates make out the bulk of the dust mass in both stars. It is likely that a significant fraction of the dust is carbon-rich. We find strong evidence for a 20–30 µm band which we tentatively attribute to FeO