Irregular particles in comet C/1995 O1 Hale-Bopp inferred from its mid-infrared spectrum

An analysis of the Infrared Space Observatory (ISO) infrared spectra of comet C/1995 O1 Hale-Bopp has been conducted. The particles in the coma are assumed to be irregular aggregates that are built by a diffusion-limited aggregation (DLA) procedure and are composed of olivine, both amorphous and crystalline, and glassy carbon. To simulate the morphological structure of the interstellar dust, the silicate component is placed in the inner layers of the aggregate, while the carbon dipoles occupy the outermost layers. The particle emissivities are calculated using the discrete dipole approximation (DDA) method of Draine & Flatau. The dust size distribution functions at different heliocentric distances (2.8, 2.9, and 3.9 AU) have been determined as the solution to three overdetermined systems of equations for the infrared flux at each wavelength in the 8-40 mum region. These size distributions can be well fitted by a power law, n(r) proportional to r(-alpha), having power indexes that decrease with heliocentric distance as alpha = 3.6 +/- 0.3 at 2.8 AU and -3.3 +/- 0.3 at 3.9 AU.