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

Abstract

We model the 10 µm absorption spectra of nonspherical particles composed of amorphous silicate. We consider two classes of particles, compact ones and fractal aggregates composed of homogeneous spheres. For the compact particles we consider Gaussian random spheres with various degrees of non-sphericity. For the fractal aggregates we compute the absorption spectra for various fractal dimensions. The 10 µm spectra are computed for ensembles of these particles in random orientation using the well-known Discrete Dipole Approximation. We compare our results to spectra obtained when using volume equivalent homogeneous spheres and to those computed using a porous sphere approximation. We conclude that, in general, nonspherical particles show a spectral signature that is similar to that of homogeneous spheres with a smaller material volume. This effect is overestimated when approximating the particles by porous spheres with the same volume filling fraction. For aggregates with fractal dimensions typically predicted for cosmic dust, we show that the spectral signature characteristic of very small homogeneous spheres (with a volume equivalent radius r V 0.5 µm) can be detected even in very large particles. We conclude that particle sizes are underestimated when using homogeneous spheres to model the emission spectra of astronomical sources. In contrast, the particle sizes are severely overestimated when using equivalent porous spheres to fit observations of 10 µm silicate emission.Key words. infrared: general -stars: circumstellar matter -stars: planetary systems: protoplanetary disks IntroductionThe interpretation of absorption and emission spectra observed from astronomical objects requires knowledge of the absorption cross section as a function of the dust grain characteristics, such as size, shape and composition. Usually the absorption spectra are modeled using homogeneous spherical particles for which calculations can easily be done using Mie theory (Mie 1908). Although cosmic dust grains are in general not homogeneous spheres, in some cases Mie theory calculations can reproduce the observations quite accurately (see e.g. Hansen & Hovenier 1974;Kemper et al. 2004). However, in other cases one has to find a way of modeling the effects of particle shape in order to reproduce observations or laboratory measurements (see e.g. Mishchenko et al. 2000). It is therefore important to know the effects of the adopted particle shape model on the derived dust parameters, such as the particle size and structure (see e.g. Fabian et al. 2001;Min et al. 2003Min et al. , 2005a.From different formation mechanisms different types of particles may form. For example, when dust grains form from direct gas phase condensation, compact particles may be created. Alternatively, when dust grains stick together to form larger particles, complex aggregated structures may be formed. We study particles in both classes using irregularly shaped compact particles and fractal aggregates. For the compact particle shapes we use so-called Gaussian...

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“…These spectra are flat and do not show any silicate emission band. We refer to a study of Min et al (2006), where it was found that the silicate band disappears if the averaged dust grain size exceeds ∼4 μm.…”

Infrared emission from the composite grains: effects of inclusions and porosities on the 10 and 18 μm features

“…O'Donnell (1994) also did not find any shift in the 10 μm or 18 μm features for the grains containing silicates with the inclusions of carbons (glassy and amorphous). Min et al (2006Min et al ( , 2007 have used DDA to study the composite and aggregated silicates and found that the 10 μm feature shifts to the shorter wavelengths. Jones (1988) found enhancement in the infrared absorption features at 9.7 μm and 18 μm for porous silicate grains and hollow spheres.…”

Dust Opacities

“…The 10 µm silicate feature of aggregates of different fractal dimension was studied in detail by Min et al (2006a). The conclusion of this study is that for the same aggregate mass (so the same r V ) the feature is stronger when the fluffyness of the aggregate is increased (i.e.…”

“…Fig. 11 (taken from Min et al 2006a) displays this very clearly. The way to understand this is that the particle needs to become 'optically thick' for the feature to decrease in strength.…”

“…Below we will discuss how the different size indicators connect to these two different scales in an aggregate particle. These insights are based on studies by Kataoka et al (2014) Kimura et al (2006);Min et al (2006a);Volten et al (2007) and DDA computations of aggregate particles from Min et al in prep. Kataoka et al (2014) discuss the opacity evolution of very fluffy aggregates with particle size and filling factor using effective medium theory.…”