Retrieving Dust Grain Sizes from Photopolarimetry: An Experimental Approach

TL;DR

This study experimentally characterizes the scattering and polarization properties of cosmic dust analogs, revealing limitations of spherical models like Mie theory in accurately retrieving dust grain sizes and compositions.

Contribution

The paper provides experimental phase function and polarization data for forsterite dust particles and demonstrates the inaccuracies of spherical models in interpreting polarimetric observations.

Findings

Phase function behavior indicates size regime.

Maximum DLP position depends on particle size.

Spherical models produce significant errors in size and composition estimates.

Abstract

We present the experimental phase function, degree of linear polarization (DLP), and linear depolarization (deltaL) curves of a set of forsterite samples representative of low-absorbing cosmic dust particles. The samples are prepared using state-of-the-art size-segregating techniques to obtain narrow size distributions spanning a broad range of the scattering size parameter domain. We conclude that the behavior of the phase function at the side- and back-scattering regions provides information on the size regime, the position and magnitude of the maximum of the DLP curve are strongly dependent on particle size, the negative polarization branch is mainly produced by particles with size parameters in the approx. 6 to 20 range, and the deltaL is strongly dependent on particle size at all measured phase angles except for the exact backward direction. From a direct comparison of the experimental data with computations for spherical particles, it becomes clear that the use of the spherical model for simulating the phase function and DLP curves of irregular dust produces dramatic errors in the retrieved composition and size of the scattering particles: The experimental phase functions are reproduced by assuming unrealistically high values of the imaginary part of the refractive index. The spherical model does not reproduce the bell-shaped DLP curve of dust particles with sizes in the resonance and/or geometric optics size domain. Thus, the use of the Mie model for analyzing polarimetric observations might prevent locating dust particles with sizes of the order of or larger than the wavelength of the incident light.