Composite dust grains: Modeling of infrared absorption bands

We investigate the influence of small-scale asphericity of the surfaces of dust grains on the characteristics of the two deepest absorption bands observed in the spectra of protostellar objects and stars (the 3.1 μm water-ice and 9.7 μm silicate bands). The model used has composite particles in the form of radially inhomogeneous spheres with intermediate layers in which the index of refraction changes. The observed band widths and the ratios of the optical depths at the band centers can be explained if the grains are composed of small particles consisting of silicate cores with thin ice mantles and rough surfaces. The grain surface roughness considerably broadens the profile of the silicate band.