We have studied the effects of porosity on the extinction of the silicate and graphite grains in the spectral range of -. For these calculations the discrete dipole approximation is used because it takes into consideration the inhomogeneous structure (porosity, surface roughness) within the grain (Wolff et al., 1994 & 1998). The extinction curves for the porous graphite grains show the shift in the central wavelength of the extinction peak as well as variation in the width of the peak, with the porosity. These results on the porous grains indicate that the structure of the grains plays an important role in the interstellar extinction and needs to be studied in more details. We found that the porous grain models reproduce the average observed extinction reasonably well. The visual albedo for the porous grain model is about 0.6 which is found to be consistent with the observations. The application of DDA (or the validity criteria for DDA) however poses a computational challenge particularly for large values of the size parameter and the refractive index , since greater number of dipoles N are required; which would in turn require large computer memory and considerable cpu time. The effective medium theory with Mie-type series solutions, may still play an important role in deriving the properties of interstellar dust (Wolff et al., 1998). However, before applying the EMT, the accuracy and the range of applicability of several mixing rules need to be determined (Chylek et al., 1988 and Ossenkopf, 1991). It would be very useful to compare the DDA scattering properties for porous grains (with a range of porosity) with those computed by the EMT/series solution technique in order to examine the applicability of several mixing rules. A synthetic approach combining the laboratory data on the porous and fluffy particles (Gustafson, 1996 and Zerull et al., 1993) with the theoretical calculations would also help greatly to interpret the observed interstellar extinction and polarization.
© European Southern Observatory (ESO) 1999
Online publication: July 26, 1999