Abstract. 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 for determining 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 small ( < ∼ 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 with 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.

In many models of dusty objects in space the grains are assumed to be composite or fluffy. However, the computation of the optical properties of such particles is still a very difficult problem. We analyze how the increase of grain porosity influences basic features of cosmic dustinterstellar extinction, dust temperature, infrared bands and millimeter opacity. It is found that an increase of porosity leads to an increase of extinction cross sections at some wavelengths and a decrease at others depending on the grain model. However, this behaviour is sufficient to reproduce the extinction curve in the direction of the star σ Sco using current solar abundances. In the case of the star ζ Oph our model requires larger amounts of carbon and iron in the dust-phase than is available. Porous grains can reproduce the flat extinction across the 3−8 µm wavelength range measured for several lines of sight by ISO and Spitzer. Porous grains are generally cooler than compact grains. At the same time, the temperature of very porous grains becomes slightly larger in the case of the EMT-Mie calculations in comparison with the results found from the layered-sphere model. The layered-sphere model predicts a broadening of infrared bands and a shift of the peak position to larger wavelengths as porosity grows. In the case of the EMT-Mie model variations of the feature profile are less significant. It is also shown that the millimeter mass absorption coefficients grow as porosity increases with a faster growth occurring for particles with Rayleigh/non-Rayleigh inclusions. As a result, for very porous particles the coefficients given by two models can differ by a factor of about 3.

Abstract. The database we announce contains references to the papers, data files and links to the Internet resources related to measurements and calculations of the optical constants of the materials of astronomical interest: different silicates, ices, oxides, sulfides, carbides, carbonaceous species from amorphous carbon to graphite and diamonds, etc.We describe the general structure and content of the database which has now free access via Internet:

We extend and investigate the spheroidal model of interstellar dust grains used to simultaneously interpret the observed interstellar extinction and polarization curves. We compare our model with similar models recently suggested by other authors, study its properties and apply it to fit the normalized extinction A(λ)/AV and the polarizing efficiency P(λ)/A(λ) measured in the near‐infrared to far‐ultraviolet region for several stars seen through one large cloud. We conclude that the model parameter Ω being the angle between the line of sight and the magnetic field direction can be more or less reliably determined from comparison of the theory and observations. This opens a way to study the spatial structure of interstellar magnetic fields by using multiwavelength photometric and polarimetric observations.

No abstract

No abstract

We use the separation of variables and T-matrix methods to calculate the optical properties of homogeneous spheroids with refractive indices from m = 1.3 + 0.0i up to 3 + 4i.It is found that the extinction cross-sections for highly absorbing spheroids are normally 1.5 -2 times larger than those for spheres of the same volume. The albedo of the non-spherical particles rather slightly depends on the particle shape and is mainly determined by the imaginary part of the refractive index. Beginning at some size, the spheroidal particles do not polarize the transmitted radiation independent of their shape.We also suggest a new approach for axisymmetric particles which would combine the strong aspects of both methods mentioned above and give several values of the cross-sections as benchmarks in tabular form.

No abstract