Comparative analysis of polarimetric signatures of aligned and optically active ('homochiral') dust particles

摘要

We model light scattering by aligned particles and particles that contain homochiral organics, i.e. organics that possess optical activity (circular birefringence and circular dichroism), to check for a method to distinguish between these particles based on their linear and circular polarization. For aligned particles, we consider alignment in a magnetic field that aligns the particles with the longest particle dimension perpendicular to the magnetic field lines. We model those particles as polydisperse ensembles of prolate and oblate spheroids made of ice, silicate and cosmic organics. We model optically active particles as aggregates of submicron monomers made of chlorophyll, which has optical constants with values similar to those of silicates but possesses distinct optical activity in the visible. The results of the modeling show that alignment and optical activity produce a rather similar shape of phase angle dependences of linear and circular polarization, making it difficult to distinguish between them. However, a difference was found in the exact backscattering and forward scattering directions, where aligned particles have non-zero linear polarization and zero circular polarization whereas optically active particles have non-zero circular polarization and zero linear polarization. We also studied correlations between linear and circular polarization and found that at small phase angles both aligned and optically active particles show correlation between linear and circular polarization. However, at phase angles larger than 100°, linear and circular polarizations correlate for aligned particles and anticorrelate for optically active particles. This difference in correlations may be used to distinguish between two mechanisms of formation of circular polarization. Also, the spectral dependence of circular polarization for these two mechanisms is very different because it is defined by the refractive index for aligned particles and circular dichroism and birefringence for optically active particles. Our results can be helpful in determining the mechanism responsible for formation of circular polarization at scattering of light by dust particles in comets, star-forming regions and circumstellar disks, and by aerosols in atmospheres of exoplanets.