1 Instituut voor Sterrenkunde, KU Leuven, Celestijnen laan 200B, B3001 Heverlee, Belgium
Circumstellar disks thata ccompany the satr formation process disappear on a timescale of several million years. During this eveolution, the infrared radiation which characterizes the disk diminishes. Nevertheless, faint disks appear to survive in several main-sequence stars, such as Beta Pictoris and Vega, suggesting a reservoir of bodies the collisions of which may replenish the disk. The stars we will discuss are so-called isolated Herbig Ae/Be stars: their strong infrared excess argues fo ayoung age, but they are not located in a star forming region, therefore they can be considered as transition objects between the youngest, embeddded starrs and main-sequence objects such as Beta Pictoris. Isolated Herbig Ae/Be stars are ideal targets for infrared spectroscopy of circumstellar disks, since they are bright and their isolated nature avoids confusion with the loose surrondings that ocur for more embedded sources. A guaranteed time proposal has been carried out with ISO, obtaining SWS and LWS spectra over a wide range of objects (from embedded to main-sequence). In addition to the already published spectru of HD 100546, whihc we now modelled with a radiative tansfer code, we present the spectra of HD 142527 and HD 179218. The latter two stars are 100 times younger than HD 100546, but HD 1792218 has a similar spectrum. It also has PAH bands in the near infrared, and a lot of crystalline silicate features at longer wavelengths. We do not have an LWS spectrum, but we can clearly see that we cannot account for all the peaks using a single kind of silictae, which was the case for HD 100546 (forsterite). Here, much more pyroxenes bands have been detected. the timescale for crystallisatino differs very much from star to star, because no evidence for cold crystalline matter is present in HD 145527, though we need a hot crystalline component fro the 10 microns bump. The 30-90 K dust-component can be accounted for using three different minerals, namely cold amoprhous silicates (25%), crystalline water ice (60%), and montmorillonite (15%), a hydrous layer-lattice silicate, known to be present in IDPs. From the star-star differences, we can conclude that both density, temperature and age have a lot of influence on the dust processing. Since these stars are filling the gap between embedded YSO's and main-sequence stars with possible solar systems, more of them should and will be analysed in the future.