Tentative detection of micron-sized forsterite grains in the proto-planetary disk surrounding HD 100453^*

¹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium e-mail: bart.vandenbussche@ster.kuleuven.ac.be
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748 Garching bei München, Germany
⁴ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 10 March 2003
Accepted: 6 July 2004

Abstract

We have re-analyzed the ISO-SWS spectrum of the intermediate mass pre-main-sequence star HD 100453. We confirm the weakness of the 10 μm amorphous silicate band. We have found strong indications of the presence of a crystalline silicate emission at 34 μm, which had escaped detection so far due to severe glitches in the data. The 23.5 and 28.5 μm forsterite bands have not been detected. This result indicates that the absence of crystalline silicate features at wavelengths shorter than 30 μm does not prove the absence of crystalline silicate material in the circumstellar disks of young stars. The 34 μm feature can be fitted well with crystalline forsterite grains with an average size of at least 2 μm, but the precise size is uncertain due to the poor data quality. The temperature of these grains must be 110 K or below, or otherwise the 23.5 and 28.5 micron features would be visible as well. Ground-based 10 μm 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 ≤ 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 10 μm silicate emission in some Herbig Ae stars is therefore due to the removal of small grains by dust growth processes.