The size distribution of interstellar dust particles as determined from polarization: Spheroids

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Abstract

We have determined the size distribution of polarizing interstellar dust grains based on electromagnetic scattering by spheroidal particles, extending our original work based on infinite cylinders. Shapes and alignment variants included the following: perfectly aligned oblate particles with axial ratios 1.414:1, 2:1, 4:1, and 6:1, picket fence prolate particles with axial ratios 2:1 and 4:1, and 2:1 prolate particles with perfect spinning alignment. Our analysis is based on bare silicate grains. The size distributions found are qualitatively similar to those derived using infinite cylinders. When expressed as contributions to the total mass, the distributions peak at mean size approximately 0.2 microns and are skewed, with the relative rate of decrease to larger and smaller sizes depending on lambda _max. Using infinite cylinders, the specific requirement of a reasonable fit in the infrared produces a substantial dip in the mass distribution at approximately 0.4 microns, hinting at a bimodal mass distribution. But this dip is not present when oblate and prolate particles are used. This confirms that the dip is related to the incorrect behavior of scattering for infinite cylinders in the long wavelength limit. Fitting HST ultraviolet polarization data beyond 6 microns -1 introduces an additional bump at the small size end (approximately 0.01 microns) of the distribution. This unusual feature can be traced to the sudden rise in the imaginary part of the refractive index of 'astronomical silicate.' When a 'modified astronomical silicate' is used, the size of the additional bump is much reduced, if not absent. Based both on the smoothness of the mass distribution and on the fit to the polarization curve, oblate shapes are preferred to prolate. Among the oblates, the 6:1 oblate shape gives the most satisfactory result, simply because the width of the calculated polarization curve of single-sized 6:1 oblate particles is the narrowest. Mass distributions from fitting extinction curves using aligned spheroids have been determined. They resemble closely those based on spheres. Polarization to extinction ratios are large enough to match the maximum interstellar value for all axial ratios and shapes studied, though the most spherical (1.4:1 oblate and 2:1 spinning prolate) particles would have to be nearly perfectly aligned. The birefringence and interstellar circular polarization in the ultraviolet are predicted.

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