ISO spectroscopy of circumstellar dust in the Herbig Ae systems AB Aur andHD 163296

Using both the Short- and Long-wavelength Spectrometers on board the Infrar ed Space Observatory (ISO), we have obtained infrared spectra of the Herbig Ae systems AB Aur and HD 163296. In addition, we obtained ground-based N b and images of HD 163296. Our results can be summarized as follows: (I) The main dust components in AB Am are amorphous silicates, iron oxide and PAHs; (2) The circumstellar dust in HD 163296 consists of amorphous silicates, i ron oxide, water ice and a small fraction of crystalline silicates; (3) The infrared fluxes of HD 163296 are dominated by solid state features; (4) Th e colour temperature of the underlying continuum is much cooler in HD 16329 6 than in AB Aur, pointing to the existence of a population of very large ( mm sized) dust grains in HD 163296; (5) The composition and degree of cryst allization of circumstellar dust are poorly correlated with the age of the central star. The processes of crystallization and grain growth are also no t necessarily coupled. This means that either the evolution of circumstella r dust in protoplanetary disks happens very rapidly (within a few Myr), or that this evolution is governed by factors other than stellar mass and age.