CosTuuM: polarized thermal dust emission by magnetically oriented spheroidal grains

TL;DR

CosTuuM is an open-source Python library that accurately models polarized thermal dust emission by spheroidal grains aligned with magnetic fields, improving radiative transfer simulations.

Contribution

The paper introduces CosTuuM, a novel software tool for calculating optical properties of aligned spheroidal dust grains, with validation and analysis of common approximation methods.

Findings

Linear polarization can be modeled by combining perfectly aligned and randomly oriented grains.

Pick fence alignment fails at short wavelengths.

Using a shape distribution improves accuracy over a single shape approximation.

Abstract

We present the new open source C++-based Python library CosTuuM that can be used to generate infrared absorption and emission coefficients for arbitrary mixtures of spheroidal dust grains that are (partially) aligned with a magnetic field. We outline the algorithms underlying the software, demonstrate the accuracy of our results using benchmarks from literature, and use our tool to investigate some commonly used approximative recipes. We find that the linear polarization fraction for a partially aligned dust grain mixture can be accurately represented by an appropriate linear combination of perfectly aligned grains and grains that are randomly oriented, but that the commonly used picket fence alignment breaks down for short wavelengths. We also find that for a fixed dust grain size, the absorption coefficients and linear polarization fraction for a realistic mixture of grains with various shapes cannot both be accurately represented by a single representative grain with a fixed shape, but that instead an average over an appropriate shape distribution should be used. Insufficient knowledge of an appropriate shape distribution is the main obstacle in obtaining accurate optical properties. CosTuuM is available as a standalone Python library and can be used to generate optical properties to be used in radiative transfer applications.