Polarization of cosmic dust simulated with the rough spheroid model

摘要

Cosmic dust is a polydisperse mixture of irregular, often aggregated, particles. Previous attempts have tried to simulate polarimetric properties of this dust using aggregate dust models, but it has not been possible to consider particle sizes larger than a couple of microns due to limitations of computer memory and processing power. Attempts have also been made to replace aggregates by polydisperse regular particles (spheres, spheroids, cylinders), but those models could not consistently reproduce the observed photopolarimetric characteristics. In this study, we introduce to the astronomical community the software package developed by Dubovik et al. (2006) for modeling light scattering by a polydisperse mixture of randomly oriented smooth and rough spheroids of a variety of aspect ratios. The roughness of spheroids is defined by a normal distribution of the surface slopes, and its degree depends on the standard deviation of the distribution (which is zero for smooth surface and greater than zero for rough surface). The pre-calculated kernels in the software package allow for fast, accurate, and flexible modeling of different size and shape distributions. We present our results of a systematic investigation, of polarization obtained with the rough and smooth spheroid models; we study differences in their phase angle dependence and how those differences change with the particle size distribution. We found that the difference between smooth and rough particles increases with increasing effective size parameter and affects mainly the value and position of the maximum polarization. Negative polarization was found to be typical only for silicate-like refractive indexes and only when the particles have size parameters within 2.5-25. As an example of an application of the rough spheroid model, we made computations for rough spheroids that have a size distribution and composition typical for cometary dust. We found that a mixture of porous rough spheroids made of absorbing material compositionally similar to comet Halley's dust and solid silicate spheroids, dominated by particles of size parameter 5 < x < 20, can reproduce angular and spectral characteristics of the brightness and polarization observed for cometary dust. (C) 2015 Elsevier Ltd. All rights reserved.