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We model the 10 micron absorption spectra of nonspherical particles composedof amorphous silicate. We consider two classes of particles, compact ones andfractal aggregates composed of homogeneous spheres. For the compact particleswe consider Gaussian random spheres with various degrees of non-sphericity. Forthe fractal aggregates we compute the absorption spectra for various fractaldimensions. The 10 micron spectra are computed for ensembles of these particlesin random orientation using the well-known Discrete Dipole Approximation. Wecompare our results to spectra obtained when using volume equivalenthomogeneous spheres and to those computed using a porous sphere approximation.We conclude that, in general, nonspherical particles show a spectral signaturethat is similar to that of homogeneous spheres with a smaller material volume.This effect is overestimated when approximating the particles by porous sphereswith the same volume filling fraction. For aggregates with fractal dimensionstypically predicted for cosmic dust, we show that the spectral signaturecharacteristic of very small homogeneous spheres (with a volume equivalentradius r_V<0.5 micron) can be detected even in very large particles. Weconclude that particle sizes are underestimated when using homogeneous spheresto model the emission spectra of astronomical sources. In contrast, theparticle sizes are severely overestimated when using equivalent porous spheresto fit observations of 10 micron silicate emission.Comment: Accepted for publication in A&

The 10 $\boldsymbol{\mu}$m amorphous silicate feature of fractal aggregates and compact particles with complex shapes