Dust in Astrophysical Systems: Impacts on Dynamics, Plasma Physics, and Thermochemistry

Citation

Soliman, Nadine Hany (2025) Dust in Astrophysical Systems: Impacts on Dynamics, Plasma Physics, and Thermochemistry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qszw-vm92. https://resolver.caltech.edu/CaltechTHESIS:05162025-170026255

Abstract

The role of astrophysical dust is often simplified in models of galaxy and star formation, where it is typically treated as passively tracing the gas. However, dust actively influences the dynamics, thermodynamics, and observable properties of diverse environments. This thesis explores how explicitly modeling dust grain dynamics and their interactions with gas, radiation, and electromagnetic fields alters the behavior of three key astrophysical regimes: active galactic nuclei (AGN), star-forming giant molecular clouds (GMCs), and the early Universe.

Radiation pressure on dust is widely regarded as a driver of large-scale outflows in AGN, though the dynamics of this interaction remain poorly constrained. Using radiation–dust–magnetohydrodynamic (RDMHD) simulations, we show that radiation efficiently drives supersonic, dust-laden outflows, which are unstable to resonant drag instabilities (RDIs). These instabilities generate turbulence and restructure the dust into clumpy, anisotropic forms, accounting for the torus's patchiness and producing time-variable reprocessed emission in the infrared and optical.

In GMCs, dust dynamics play a crucial role in shaping both the chemistry and thermodynamics of the gas. Leveraging the STARFORGE framework with live dust dynamics and non-equilibrium thermochemistry, we show that stellar radiation redistributes dust, reducing dust accretion during the main mass growth phases and leading to substantial abundance variations among co-natal stars. Statistically, these variations align with observational data, providing an alternative mechanism for driving abundance fluctuations in co-natal stars, beyond interpretations focused solely on post-formation processes like planet accretion. We find that, for a fixed dust mass, grain size variations significantly affect the thermodynamics, influencing local opacity, radiative transport, thermal balance, and ionization structure, thereby suppressing SFE by up to an order of magnitude.

Finally, we introduce a novel mechanism for magnetogenesis in the early Universe via radiatively accelerated, charged dust grains. This "dust battery" takes advantage of the large stopping lengths of dust grains to produce significant charge separation over large distances, thereby driving electric fields and seeding magnetic fields. Unlike conventional mechanisms (e.g., Biermann battery, Weibel instability), which rely on short-range electron-ion separation, this process operates efficiently and generates magnetic fields several orders of magnitude stronger than those produced by traditional mechanisms. We derive the underlying theory, develop a Magnetohydrodynamic-Particle-In-Cell (MHD-PIC) model, and a sub-grid model suitable for implementation in cosmological contexts.

These insights underscore the importance of incorporating dust dynamics into astrophysical models to enhance our understanding of the formation and evolution of galaxies, stars, and the interstellar medium.

Thesis Files