Abstract
Context. Observations of cold condensed clouds in the interstellar medium show an enhancement in the dust emissivity at long wavelengths. Model calculations with the discrete-dipole approximation (DDA) can explain this enhancement with the coagulation of dust particles into aggregates. Aims. We study the nature of grain-grain contacts and their effects on the aggregate optical properties. Methods. We use DDA and the T-matrix method (TMM) to calculate the absorption properties of aggregate dust grains and analyse where and why the enhancement in the emissivity occurs. Results. We find that the absorption coefficient changes with material composition and with the contact area between monomers. A larger contact area, with DDA, compared to a zero-point contact with TMM, results in an enhancement of the absorption coefficient for wavelengths where the considered material has a large value n (the real part of the refractive index). Conclusions. DDA seems to be the most realistic way of taking into account “real” inter-particle contact effects in aggregate particles.
Highlights
Far-infrared (FIR) observations of dust in the interstellar medium (ISM) suggest differences in grain properties between cold condensed clouds and the diffuse ISM
We show results obtained with discrete-dipole approximation (DDA) and T-matrix method (TMM)
Our results show that the increase in emissivity for aggregates calculated using DDA is the result of larger contact areas between the monomers, compared to TMM and generalized multi-particle Mie theory (GMM)
Summary
Far-infrared (FIR) observations of dust in the interstellar medium (ISM) suggest differences in grain properties between cold condensed clouds and the diffuse ISM. Compared to spherical particles, extended aggregate structures show a stronger emission in the FIR (Wright 1987; Bazell & Dwek 1990; Fogel & Leung 1998; Stepnik et al 2003) while at short wavelengths the emission of aggregate particles is similar to that of spherical grains This effect was observed when the absorption coefficients of hollow spheres and porous particles calculated with effective medium theories were compared to compact spherical grains (Jones 1988). While for TMM the monomers are perfectly spherical, the surface of the DDA monomers can deviate significantly from perfect spheres, depending on the number of dipoles This results in different connections between monomers when they are in aggregates. Note that we do not mix monomer compositions and our aggregates are composed of either pure silicate or pure carbon monomers
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