CosTuuM: Polarized Thermal Dust Emission by Magnetically Oriented Spheroidal Grains

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.

Using the Starlight Polarization Efficiency Integral to Constrain Shapes and Porosities of Interstellar Grains

“…TESTING THE MPFA At wavelengths λ > 8µm, submicron grains have 2πa eff /λ 1, and the MPFA (Eqs. 11, 12) is known to be accurate, as verified by Vandenbroucke et al (2020). However, at "optical" wavelengths where starlight polarization is measured, 2πa eff /λ 1, and the accuracy of the MPFA must be checked by comparison to the exact averages in Eq.…”

“…The MPFA is equally simple, and leads to similar equations, but allows us to explicitly discuss polarization by a population of partially-aligned, spinning and precessing grains. While the MPFA has been used previously (Lee & Draine 1985;Draine & Allaf-Akbari 2006;Draine & Fraisse 2009;Vandenbroucke et al 2020), it is helpful to review it here.…”

Polarised emission from aligned dust grains in nearby galaxies: Predictions from the Auriga simulations

“…As in Vandenbroucke et al (2020), our assumed dust model consists of two components: (partially) aligned silicate grains and non-aligned graphite grains. Additional grain materials could be included, if the necessary material properties required to compute optical properties were available.…”

“…We furthermore assume that the silicate grains consist of spheroidal grains with a CDE2 shape distribution (Draine & Hensley 2021). As in Vandenbroucke et al (2020), we assume that only grains with x = 0.0 sizes a > 0.1 µm are aligned, and we parameterise the alignment using a single linear alignment parameter, f S,A . The composition of the dust mixture itself is governed by a second linear mixture parameter, f G that encodes the fraction of the dust grains contained in the graphite component.…”

“…We use the Auriga suite of Milky Way type spiral galaxies (Grand et al 2017) as an input model for radiative transfer simulations using the radiative transfer code SKIRT (Baes et al 2011;Camps & Baes 2015 that self-consistently model the thermal emission by interstellar dust grains. We use an accurate model for the optical properties of spheroidal dust grains, computed using the open-source package CosTuuM (Vandenbroucke et al 2020). We constrain our model based on the Planck observations of the Milky Way at 353 GHz (Planck Collaboration XIX 2015;Planck Collaboration XII 2020), and then use this same model to make predictions for nearby galaxies, observed from different inclination angles and in different broad bands.…”