Infrared Observations of Herbig Ae/Be Protoplanetary Disks by Dr. Mario van den Ancker (CfA)   I present an analysis of solid-state features in intermediate-mass young stars, based upon 3-20 micron spectra of 48 Herbig Ae/Be stars. The presence and strength of the UIR emission bands, amorphous silicate bands and crystalline silicate bands are compared to other indicators of circumstellar material, such as gaseous emission lines, photometric variability and submm photometry. The correlation between these different indicators is weak, if present at all, in our sample. However, a strong dependence on spectral type of the central star seems to be present: Stars with spectral type earlier than B9 show either amorphous silicate in absorption or infrared spectra dominated by PAH emission, whereas more than 70% of the stars of later spectral type show silicate emission. This difference can be explained by assuming that the infrared spectrum of Herbig Be stars is in general dominated by their circumstellar envelope, whereas the more slowly evolving Herbig Ae stars show a spectrum that is dominated by a disk that is passively heated by the central star. Crystalline silicates, as are also found in comets in our own solar system, are visible in 15% of the late-type Herbig stars, all of systems that are located close to the zero-age main-sequence in the HRD. These systems form a close analogue to the young solar system and may provide the strongest clue to date that the same processes that have led to the formation of rocky planets in our own solar system are also taking place around other stars.