2004A&A...427..519V

We have re-analyzed the ISO-SWS spectrum of the intermediate mass pre-main-sequence star HD 100453. We confirm the weakness of the 10µm amorphous silicate band. We have found strong indications of the presence of a crystalline silicate emission at 34µm, which had escaped detection so far due to severe glitches in the data. The 23.5 and 28.5µm forsterite bands have not been detected. This result indicates that the absence of crystalline silicate features at wavelengths shorter than 30µm does not prove the absence of crystalline silicate material in the circumstellar disks of young stars. The 34µm feature can be fitted well with crystalline forsterite grains with an average size of at least 2µm, but the precise size is uncertain due to the poor data quality. The temperature of these grains must be 110K or below, or otherwise the 23.5 and 28.5 micron features would be visible as well. Ground-based 10µm spectra show prominent emission from Polycyclic Aromatic Hydrocarbons and a weak silicate band whose shape suggests that it is dominated by a few micron size grains, of unknown lattice structure. The absence of any significant forsterite structure in the 10 micron region limits the mass of warm forsterite grains with sizes less than a few microns to ≤2x10^–4 of the mass of cold forsterite. Forsterite may be present in the warm regions, but then must be contained in even larger grains. The absence of 10µm silicate emission in some Herbig Ae stars is therefore due to the removal of small grains by dust growth processes.