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Next: Computational methods Up: Dust opacities Previous: Grain sizes and compositional

Grain structure and topology

It becomes evident from the theoretical investigations and laboratory experiments that the dust agglomeration is an efficient process in dense and relatively cold environments, like protostellar cores or protoplanetary discs (e.g., Blum et al. [2002]; Kempf, Pfalzner, & Henning [1999]; Kesselman [1980]; Nuth & Berg [1994]; Ossenkopf & Henning [1994]; Wurm & Blum [1998], [2000]). Agglomeration leads to the formation of irregular particles consisting of hundreds or thousands of tiny subgrains. Usually, dust aggregates of two extreme kinds are considered, depending on the assumed coagulation processes, namely, PCA (particle-cluster aggregation) and CCA (cluster-cluster aggregation). As the laboratory and theoretical studies reveal, the PCA aggregates are sphere-like particles having fractal dimension of about 3. They have a compact “core'' and a more rarefied “mantle''. The CCA process results in the formation of very filamentary grains with complicated structure. They have fractal dimension of roughly 2 (Stognienko, Henning, & Ossenkopf [1995]). It is worthwhile to mention that the interplanetary dust particles have a structure similar to that of the laboratory analogs, the PCA (Rietmeijer & Nuth [2000]).

During the evolution of parent objects, chemical and physical processes can further modify the properties of dust grains. For instance, accretion of volatile materials on dust surfaces and their subsequent chemical processing are efficient in outer regions of protoplanetary discs and in protostellar clouds (e.g. Aikawa et al. [1999]; Brown, Charnley, & Millar [1988]; Greenberg [1967]; Hartquist & Williams [1990]; Hasegawa & Herbst [1993]; Willacy, Rawlings, & Williams [1994]). This results in well-defined ``core-mantle'' or, more probably, ``onion-like'' grain structure. In protostellar discs, dust can be transported by the accretion flow toward hotter regions, where their volatile mantle materials evaporate, and sputtering, annealing, combustion, and crystallization processes may change the structure, composition, and sizes of the grains (Gail [2001,2002]). Therefore, it seems obvious that the real astronomical grains must have a very complicated structure and topology.

Unfortunately, modern computational methods and facilities allow only the consideration of somewhat simplified (but still reasonable) kinds of dust grains. In the present study, we focus on the following particle types:

  1. Homogeneous and composite aggregates;
  2. Homogeneous, composite, and porous composite spherical particles;
  3. Multishell and porous multishell spherical particles;

An aggregate dust particle is assumed to be a cluster of small spherical subgrains sticked together. A particle is called ``homogeneous'' if it consists of only one dust component. On the contrary, “composite'' means that a particle incorporates a fine mixture of various materials (heterogeneous particle). In addition to these two extreme cases of the chemical dust structure, we consider a model of “multishell'' grains, where each particle includes all constituents distributed within concentric spherical shells. To study the influence of the porosity on the extincting properties of dust grains, we fill composite and multishell spherical particles with vacuum. It is reasonable to think that the optical behavior of these porous multishell and porous composite particles may resemble that of more realistic kinds of dust grains.


Table: Mass fraction and density of dust components

     
Material NRM IRS IPS
       
Olivine 2.64 10–3 (3.49 gcm–3) 3.84 10–3 (3.59 gcm–3) 6.30 10–4 (3.20 gcm–3)
Iron 1.26 10–4 (7.87 gcm–3) - 7.97 10–4 (7.87 gcm–3)
Pyroxene 7.70 10–4 (3.40 gcm–3) 4.44 10–5 (3.42 gcm–3) 1.91 –3 (3.20 gcm–3)
Troilite 7.68 10–4 (4.83 gcm–3) 3.80 10–4 (4.83 gcm–3) 7.68 10–4 (4.83 gcm–3)
Refractory      
organics 3.53 10–3 (1.50 gcm–3) 3.53 10–3 (1.50 gcm–3) 3.53 10–3 (1.50 gcm–3)
Volatile      
organics 6.02 10–4 (1.00 gcm–3) 6.02 10–4 (1.00 gcm–3) 6.02 10–4 (1.00 gcm–3)
Water ice 5.55 10–3 (0.92 gcm–3) 5.55 10–3 (0.92 gcm–3) 5.55 10–3 (0.92 gcm–3)


next up previous
Next: Computational methods Up: Dust opacities Previous: Grain sizes and compositional
Dimitri Semenov 2003-03-10