1998A&A...332L..25M

It is generally assumed that planets form in the dusty disks that surround young stars (Beckwith & Sargent 1996Natur.383..139B). The Infrared Space Observatory (Kessler et al., 1996A&A...315L..27K) now enables us to determine the characteristics of these disks with unprecedented spectral resolution and signal-to-noise. We present here ISO spectra of the disk that surrounds the young star HD 100546. A remarkable variety of emission features of carbon- and oxygen-rich dust occurs. Most prominent are a series of emission features that can be attributed to silicates in crystalline form, mostly forsterite. In the interstellar medium and HII regions the silicate dust is mostly amorphous, but crystalline silicates are found in comets, meteorites and interplanetary dust particles. The forsterite features of HD 100546 are astonishingly similar to those observed in the ISO spectrum of Comet Hale-Bopp (Crovisier et al., 1997Sci...275.1904C), strengthening the hypothesis that the disk around HD 100546 contains a huge swarm of comets (Grady et al., 1997ApJ...483..449G). We argue that the crystallisation process occurs during the early evolution of the circumstellar disks of young stars and speculate about the formation of an Oort cloud around HD 100546.