MANTA-Ray: Supercharging Speeds for Calculating the Optical Properties of Fractal Aggregates in the Long-wavelength Limit

• Published in: Monthly notices of the Royal Astronomical Society 2024-12, Vol.535 (2), p.1964-1978
• Main Authors: Lodge, M G, Wakeford, H R, Leinhardt, Z M
• Format: Article
• Language: English
• Subjects: Absorption cross sections; Absorptivity; Atmospheric correction; Computing time; Dipoles; Dust; Fractal analysis; Optical properties; Particle shape; Planet formation; Planetary atmospheres; Protoplanetary disks; Refractivity
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stae2451

ABSTRACT Correctly modelling the absorptive properties of dust and haze particles is of great importance for determining the abundance of solid matter within protoplanetary discs and planetary atmospheres. Rigorous analyses such as the discrete dipole approximation (DDA) can be used to obtain accurate absorption cross-sections, but these require significant computing time and are often impractical to use in models. A simple analytical equation exists for spherical particles in the long-wavelength limit (where the wavelength is much larger than the size of the dust particle), but we demonstrate that this can significantly underestimate the absorption. This effect is found to depend strongly on refractive index, with values of $m=1+11$i corresponding to an underestimate in absorption by a factor of 1000. Here we present MANTA-Ray (modified absorption of non-spherical tiny aggregates in the RAYleigh regime): a simple model that can calculate absorption efficiencies within 10–20 per cent of the values predicted by DDA, but $10^{13}$ times faster. MANTA-Ray is very versatile and works for any wavelength and particle size in the long wavelength regime. It is also very flexible with regards to particle shape, and can correctly model structures ranging from long linear chains to tight compact clusters, composed of any material with refractive index 1 + 0.01i $\le m \le$ 11 + 11i. The packaged model is provided as publicly available code for use by the astrophysical community.