From small dust to micron-sized aggregates: The influence of structure and composition on the dust optical properties

Context. Models of astrophysical dust are key to understanding several physical processes, from the role of dust grains as cooling agents in the interstellar medium (ISM) to their evolution in dense circumstellar discs, explaining the occurrence of planetary systems around many stars. Currently, most models aim to provide optical properties for dust grains in the diffuse ISM, and many do not account properly for complexity in terms of composition and structure when dust is expected to evolve in dense astrophysical environments. Aims. Our purpose is to investigate, with a pilot sample of micron-size dust grains, the influence of hypotheses made about the dust structure, porosity, and composition when computing the optical properties of grown dust grains. We aim to produce a groundwork for building comprehensive yet realistic optical properties that accurately represent dust grains as they are expected to evolve in the dense clouds, cores, and discs. We are especially interested in exploring these effects on the resulting optical properties in the infrared and millimetre domains, where observations of these objects are widely used to constrain the dust properties. Methods. Starting from the small dust grains developed in the THEMIS 2.0 model, we used the discrete dipole approximation to compute the optical properties of 1 μm grains, varying the hypotheses made about their composition and structure. We looked at the dust scattering, emission, and extinction to isolate potential simplifications and unavoidable differences between grain structures. Results. We note significant differences in the optical properties depending on the dust structure and composition. Both the dust structure and porosity influence the dust properties in infrared and millimetre ranges, demonstrating that dust aggregates cannot be correctly approximated by compact or porous spheres. In particular, we show that the dust emissivity index in the millimetre can vary with fixed grain size. Conclusions. Our work sheds light on the importance of taking the dust structure and porosity into account when interpreting observations in astrophysical environments where dust grains may have evolved significantly. For example, measuring the dust sizes using the emissivity index from millimetre observations of the dust thermal emission is a good but degenerate tool, as we observe differences of up to 25% in the dust emissivity index with compact or aggregate grains, varying in composition and structure. Efforts in carrying out physical models of grain growth, for instance, are required to establish realistic constraints on the structure of grown dust grains, and will be used in the future to build realistic dust models for the dense ISM.