Most analyses of the extinction, absorption and scattering efficiencies of interstellar dust grains use the Mie theory, which solves for the exact electromagnetic fields in a spherical or cylindrical dust grain. But dust grains is the interstellar medium are very unlikely to be spherical, and the most likely situation is one where dust grains can collide and stick, or collide and shatter, leading to a distribution of sizes and a random distribution of shapes. In order to model such a situation, I have made randomly shaped grains in a computer using a random collision process.
Legend: Fractal dust grains are created from the coagulation of smaller subunits, starting with monomers. Two monomers collide and stick to make a dimer. The figure above shows a process where two dimers collide to make a tetramer, two tetramers collide to make an octamer, and so on. This produces what I call CL fractals.
An alternate process would collide a monomer with a dimer, giving a trimer, which then collides with another monomer to give a tetramer. This process of adding one monomer at a time continues giving a DLA fractal. The figure below shows a DLA fractal, a solid sphere, and a CL fractal, all having the same mass.
A 53 kB color GIF picture of a CL
fractal made from 1024 monomers is available.
A 35 kB color stereogram GIF of a CL fractal made from 1024 monomers is available as well. Cross your eyes to merge the two images and you will get a 3-D picture of the structure.
A 63 kb color GIF of a DLA fractal
made from 1024 monomers.
A 54 kB color stereogram GIF of a DLA fractal made from 1024 monomers is also available. Cross your eyes to merge the two images and you will get a 3-D picture of the structure.
If you have a fast link, look at a 1 MB animation of rotating fractal dust grains.
Relevant Publications
Wright, E. L. 1982, ApJ, 255, 401. "Thermalization of Starlight by Elongated Grains: Can the Microwave Background have been Produced by Stars?"
Wright, E. L. 1987, ApJ, 320, 818. "Long Wave Abs. by Fractal Dust Grains"
Wright, E. L. 1988, Nature, 336, 227. "The Ultraviolet Extinction from Graphitic Onions"
Wright, E. L. 1989, ApJ, 346, L89. "Fractal Dust Grains around R CrB Stars"
