It is highly unlikely that the interstellar grains have regular shapes (i.e. spherical, cylindrical or spheroidal) or that they are homogeneous in composition and structure. The collected interplanetary particles are highly porous and consist of loosely aggregated collections of sub-grains (Brownlee, 1987). Mathis & Whiffen (1989), Henning & Stognienko (1993), Wolff et al., (1994) and Bazell & Dwek (1990) have considered that interstellar grains too have this morphology and composition. Recently, in order to overcome the `interstellar carbon crisis' (or the abundance constraints) Mathis (1996) and Dwek (1997) have proposed a composite fluffy dust model. The optical properties of these porous and fluffy particles should be quite different from those of solid homogeneous particles. Unfortunately, it is not yet possible to rigorously treat the absorption and scattering of light by porous (inhomogeneous) or irregularly shaped particles. In general, there are two approaches to calculate optical properties of the inhomogeneous (porous) particles. One approach is to use the finite element method. e.g. Discrete Dipole Approximation (DDA). The second approach is the application of the effective medium theories (EMT's). In EMT the inhomogeneous particle is replaced by a homogeneous one with some averaged `effective' dielectric function (for discussion on EMT see e.g. Bohren & Huffman, 1983). The effects related to the fluctuations of the dielectric function within the inhomogeneous structures such as surface roughness and special distributions of the components can not be treated by the averaging approach of the EMT's. The discrete dipole approximation (DDA) allows the consideration of irregular shape effects and special distributions of the components in the particles (Wolff et al., 1994; Wolff et al., 1998).
In our earlier paper (Vaidya & Gupta 1997, hereafter Paper I) using DDA we had obtained the extinction efficiencies of the porous silicate and graphite grains and we had evaluated the interstellar extinction curve between and . These results clearly showed that the extinction properties of the grains are modified if the grains are porous. This motivated us to study the extinction properties of the porous silicate and graphite grains in the wavelength range between to in the infrared and from to in the ultraviolet region and to evaluate the interstellar extinction curve (i.e. E(-V)/E(B-V) vs ) in the entire wavelength range of 0.10 to . We have also evaluated the albedo, i.e. the ratio of for the porous grains in the UV and the optical region.
In Sect. 2 we give the validity criteria for the DDA and the porous grain models. In Sect. 3 we present the results of our computations and discuss these results. The main conclusions of our study are given in Sect. 4.
© European Southern Observatory (ESO) 1999
Online publication: July 26, 1999