Letter to the Editor

Spectroscopic diagnostic for the mineralogy of large dust grains

Astronomical institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Corresponding author: M. Min, mmin@science.uva.nl

Received: 31 October 2003
Accepted: 27 November 2003

Abstract

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 μm virtually disappear for grains larger than about 10 μm. 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.