Effects of electromagnetic interaction in the polarization of light scattered by cometary and other types of cosmic dust

摘要

Context. We study how the electromagnetic interaction between the monomers in aggregates affects the polarization of cosmic dust. Aims. We aim to show that the electromagnetic interactiondepends on the porosity and composition of the aggregates andcontributes significantly to the spectral gradient of polarization(polarimetric color). The results may explain the observations of somecomets that demonstrated atypical negative polarimetric color in thevisible and also a reverse of the positive polarimetric color to thenegative one in the near-infrared. Methods. We performed computer simulations of the lightscattering by aggregates consisting of spheres made of a variety ofmaterials: transparent, absorptive, and the material similar to that ofthe dust in comet Halley. We studied how the number of monomers coveredby the electromagnetic wave at a single period (on the light path equalto one wavelength) affects their interaction by considering linearclusters of 2 and 10 monomers of radius of 0.1 .Results. Electromagnetic interaction between the monomers inaggregates depolarizes the light. The interaction becomes stronger ifmore monomers are covered by the electromagnetic wave at a singleperiod. Thus, the porosity of aggregates influences their polarization.The electromagnetic interaction also depends on composition and isstronger for transparent materials. Conclusions. Electromagnetic interaction between the monomers inaggregates may explain why the polarimetric color of comet dustdecreases as observations move from the visible to the near-infraredsince a longer wavelength covers more monomers. It may also explain whysome comets exhibit negative polarimetric color even in the visible;these comets may have more compact dust. Strong electromagneticinteraction resulted either from compactness or transparency of thematerial can explain the negative polarimetric color of interplanetarydust and debris disks and contribute to the polarization of asteroids.In general, the spectral dependence of polarization is a promising toolfor studying the properties of cosmic dust particles, particularlytheir porosity. Key words: scattering - techniques: polarimetric - methods:numerical - comets: general - minor planets, asteroids: general -interplanetary medium