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Implementing Non-Spherical Grain Shape and Size Distributions into Radiative Transfer Code DUSTY: Application to AGB Stars

Presentation #234.01 in the session Dust.

Published onJul 01, 2023
Implementing Non-Spherical Grain Shape and Size Distributions into Radiative Transfer Code DUSTY: Application to AGB Stars

As main-sequence stars begin to die, they spew gases into space. These gases cool and dust can condense, enriching the interstellar medium and becoming the building blocks for planets, asteroids, planetary nebulae, etc. There is dust in almost every direction we look in space and as far back in time as we can see (e.g., Bertoldi et al. 2003, Beelen et al. 2006, Hines et al. 2006, Michalowski et al. 2010). Cosmic dust grains come in many different compositions, crystal structures, shapes, and sizes; all of these parameters affect how light interacts with dust. Most radiative transfer (RT) codes assume dust grains are spherical for simplicity. Most dust grains are not spheres. The shapes and sizes of dust grains (along with their composition and crystal structure) affect their interactions with light. Thus, models that oversimplify the morphologies of dust grains will not be able to adequately represent real dusty environments. We can solve this by creating a new, versatile code that creates absorption and scattering cross-sections that can be used in RT models. We would like to build an open-source python code that allows users to generate wavelength-dependent absorption and scattering coefficient files for grains of multiple compositions, crystal structures, size and shape distributions. This would initially be formatted for the one-dimensional RT code DUSTY, which uses simple ascii files for input and thus could easily be adapted for input into other RT codes. We will apply our code to 3 specific grain materials: forsterite (Mg2SiO4), silicon carbide (SiC), and corundum (Al2O3). The effect of grain shape in the Rayleigh regime for a single grain volume has already been calculated for these materials and have been compared to features observed in astrophysical environments (e.g., Corman, 2010, Min, Hovenier, & de Koter, 2003, DePew, Speck, & Dijkstra, 2006), providing a benchmark against which to test our code. We intend to extend this analysis beyond the Rayleigh regime and into the geometric scattering regime. Preliminary code has already been written that produces the scattering and absorption cross-sections for a simple continuous distribution of ellipsoids for a single grain volume and composition. This will be taken further as we provide options specifying grain shape distribution, grain size distributions, and compositional options. We seek to present our preliminary findings comparing how different axial measurements of forsterite are affected by different size and shape distributions.

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