THE SPECTRUM OF THE YOUNG STAR HD-100546 OBSERVED WITH THE INFRARED SPACE-OBSERVATORY

It is generally assumed that planets form in the dusty disks that surr ound young stars (Beckwith & Sargent 1996). The Infrared Space Observa tory (Kessler et al. 1996) now enables us to determine the characteris tics of these disks with unprecedented spectral resolution and signal- to-noise, We present here ISO spectra of the disk that surrounds the y oung star HD 100546. A remarkable variety of emission features of carb on-and oxygen-rich dust occurs. Most prominent are a series of emissio n features that can be attributed to silicates in crystalline form, mo stly forsterite. In the interstellar medium and Hn regions the silicat e dust is mostly amorphous, but crystalline silicates are found in com ets, meteorites and interplanetary dust particles. The forsterite feat ures of HD 100546 are astonishingly similar to those observed in the I SO spectrum of Comet Hale-Bopp (Crovisier et al. 1997), strengthening the hypothesis that the disk around HD 100546 contains a huge swarm of comets (Grady et al. 1997). 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 10 0546.