A comprehensive model of morphologically realistic Cosmic Dust particles: an application to mimic the unusual Polarization properties of the interstellar Comet 2I/Borisov

Abstract

The cosmic dust particles found in space are mainly porous aggregates of smaller grains. Theoretically, these aggregates are replicated using fractal geometry, assuming a cluster of spheres. Although, the light scattering response of cosmic dust aggregates has been thoroughly studied using clusters of spherical grains in the past few decades, yet, the effect of irregularities on the surface of each grain in an entire aggregate has mostly been neglected. We, for the first time, introduce a visually realistic cosmic dust model which incorporates a mixture of rough fractal aggregates (RFA) and agglomerated debris (Solids) to replicate the unusual polarization-phase curve observed in case of the interstellar comet 2I/Borisov at multiple wavelengths. The authenticity of the RFA structures has been verified by replicating light scattering results of circumstellar dust analogues from the Granada Amsterdam Light Scattering Database. We demonstrate that the light scattering response from the RFA structures has a very close resemblance with the experimental values. Finally, we model the observed polarization-phase curve of the interstellar comet 2I/Borisov using a mixture of RFA and solid particles. The best-fit data indicates presence of higher percentage of porous RFA structures 80% owing to the fact that the comet carries higher percentage of small and highly porous pristine cosmic dust particles. Further, the model indicates that the unusually steeper polarimetric slope and the high dust-to-gas ratio in relatively newer comets is mainly due to higher porous-to-compact ratio.