A&A 429, 371-381 (2005)
Modelling the optical properties of composite and porous interstellar grainsN. V. Voshchinnikov1, 2, V. B. Il'in1, 2 and Th. Henning3
Sobolev Astronomical Institute,
St. Petersburg University, Universitetskii prosp. 28,
St. Petersburg 198504, Russia
Isaac Newton Institute of Chile, St. Petersburg Branch, Russia
Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
(Received 3 March 2003 / Accepted 16 August 2004)
There are indications that interstellar and interplanetary
dust grains have an inhomogeneous and fluffy structure. We investigate
different methods to describe light scattering by such composite particles.
Both a model of layered particles and discrete dipole calculations
for particles with Rayleigh and non-Rayleigh inclusions are used.
The calculations demonstrate that porosity is a key parameter
light scattering. We find that the optical properties of the layered particles depend
on the number and position of layers if the number of layers is
15). For a larger number of layers the scattering characteristics become
independent of the layer sequence. The optical properties of particles
with inclusions depend on the size of inclusions provided the porosity is large.
The scattering characteristics of very porous particles with
inclusions of different sizes are found to be close to those of multi-layered spheres.
We compare the results of these calculations
the predictions of the effective medium theories (EMT)
which are often used in astronomy as a tool to calculate the optical properties of
composite particles. The results of our analysis show that the internal structure of grains
(layers versus inclusions) only slightly affects the optics of particles provided the porosity
does not exceed 50%. It is also demonstrated that in this case the optical properties of composite grains
calculated with EMT agree with the results of the exact method for layered particles.
For larger porosity, the standard EMT rules (i.e., Garnett and Bruggeman rules)
give reliable results for particles with Rayleigh inclusions only.
dust, extinction --
© ESO 2004