Analysis of Coma Dust Optical Properties in Comet C/1995 O1 (Hale–Bopp): II. Effects of Polarization

Efforts to apply a single-scattering polarized radiative transfer code to interpret photopolarimetric measurements of coma dust optical properties in Comet Hale–Bopp corroborate previous photometrically derived conclusions concerning the predominance of small sized particles in Hale–Bopp's coma. Calculations of the degree of linear polarization (DP) as a function of observation phase angle (α) produced by prolate spheroidal crystalline olivine particles with effective radii ( a=0.216 μm) are compatible with the comet's measured polarization in standard filters at λ=0.4845 and 0.684 μm. Our rudimentary “trade-off” studies highlight the extreme sensitivity of DP to dust particle size and shape. A combination of viewing geometry effects in association with enhanced multiple scattering might provide a quantitative explanation of the negative polarization for 0°≤α≤20° seen in Hale–Bopp and other comets.