A&A 413, L35-L38 (2004)
Spectroscopic diagnostic for the mineralogy of large dust grainsM. Min, C. Dominik and L. B. F. M. Waters
Astronomical institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
(Received 31 October 2003 / Accepted 27 November 2003)
We examine the thermal infrared spectra of large dust grains composed of amorphous and crystalline olivine. Strong resonances in the refractive indices result in detectable spectral structure even when the grain is much larger than the wavelength at which it radiates. We apply this to the thermal infrared spectra of compact amorphous and crystalline silicates. The weak resonances of amorphous silicates at 9.7 and 18 m virtually disappear for grains larger than about 10 m. In contrast, the strong resonances of crystalline silicates produce emission dips in the infrared spectra of large grains; these emission dips are shifted in wavelength compared to the emission peaks commonly seen in small crystalline silicate grains. We discuss the effect of a fluffy or compact grain structure on the infrared emission spectra of large grains, and apply our results to the dust shell surrounding Vega.
Offprint request: M. Min, firstname.lastname@example.org
© ESO 2004