The T-matrix method for modeling the optical properties of grains in exoplanetary atmospheres
Observations of exoplanetary atmospheres have revealed a rather complex co-existence between the gaseous and condensed phases, ranging from atmospheres apparently covered in clouds to clear ones, across a myriad of intermediate regimes. The interaction between solids and stellar radiation is of fundamental importance when forward or retrieval models are compared to observations. The simplest approach is to assume spherical particles, whose scattering and absorption properties are readily computed with Mie theory. More advanced computational methods, however, can also treat non-spherical condensates. Among these, the transition matrix approach has proven to provide a rigorous and efficient calculation of the extinction by non-spherical particles over a broad range of sizes and refractive indices. We here focus on the absorption and scattering cross sections of forsterite, spinel and silicon monoxide from the near to the far infrared range for particles of different sizes, with shapes ranging from compact ensembles of spheres to porous aggregates. We find that, depending on the size and the configuration chosen, the differences in extinction can be as high as a factor of 2-3, with structured grains being more efficient absorbers in the near infrared for sizes greater than 1 $μ$m. In a fiducial transmission spectrum we show that in the $λ= 1-5\,μ$m range the amount of spherical material needed to match the contribution of the structured grains can be a factor 2 higher. Such calculations, available for the community through a Zenodo repository, prove to be useful for interpreting recent and future observations of exoplanetary atmospheres.