Spheroidal core-mantle particle absorption, scattering, and polarisation in the long-wavelength limit

Abstract

The numerical calculation of optical properties (extinction, absorption, scattering and polarisation efficiencies) is often time-consuming for non-spherical and inhomogeneous particles. Where possible analytical methods are therefore to be preferred. We provide an analytical tool to derive the optical properties of mantled spheroidal particles, of arbitrary axis ratio, in the long wavelength limit (a << lambda), where the mantle form may be confocal, co-axial or of constant depth with respect to the particle core. We have developed an analytical approach to spheroidal core/mantle particle optical property calculations. The analytical method compares well with DDSCAT numerical calculations and, under limited circumstances, with those made using the Bruggemann effective medium theory (EMT).The analytical method presented here provides a useful tool to explore the optical and polarisation properties of core/mantle spheroidal particles at long wavelengths (lambda >~ 8mu) and is simpler and faster to implement than corresponding numerical methods. We caution against the use of EMT methods in approximating the optical properties of core/mantle particles.

10 pages, 6 figures, accepted for publication in A&A 25/12/2025

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