Spatially and spectrally resolved 10 mum emission in Herbig Ae/Be stars

We present new mid-infrared spectroscopy of the emission from warm circumstellar dust grains in the Herbig Ae stars HD 100546, HD 97048 and HD 104237, with a spatial resolution of ≈0.9 arcsec. We find that the emission in the UIR bands at 8.6, 11.3 and (HD 97048 only) 12.7 mum is extended in the first two sources. The continuum emission is resolved in HD 97048 and possibly in HD 100546. HD 104237 is not spatially resolved in our observations. We find that the UIR emission in HD 100546 and HD 97048 is extended on a scale of (several) 100 AU, corresponding to the outer disk scale in flaring disk models. Small carbonaceous particles are the dominant source of opacity in the HD 97048 disk

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

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 mum amorphous silicate band. We have found strong indications of the presence of a crystalline silicate emission at 34 mum, which had escaped detection so far due to severe glitches in the data. The 23.5 and 28.5 mum forsterite bands have not been detected. This result indicates that the absence of crystalline silicate features at wavelengths shorter than 30 mum does not prove the absence of crystalline silicate material in the circumstellar disks of young stars. The 34 mum feature can be fitted well with crystalline forsterite grains with an average size of at least 2 mum, 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 mum 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 <= 2¿ 10-4 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 10 mum silicate emission in some Herbig Ae stars is therefore due to the removal of small grains by dust growth processes