Dust dynamics in astrophysical fluids

In this thesis, we take a close look at the one percent (by mass) of the interstellar medium (ISM) that is ignored in the vast majority of the studies involving dynamics of astrophysical fluids: dust. While only a small portion of the mass in the ISM (and most other astrophysical fluids) is stored in dust grains, their presence is highly important for a number of reasons. In a number of cases we are interested in the grains themselves, because the elements they contain may give us insight in the chemical conditions that are of importance or about the chemical enrichment that has taken place in the past. On the other hand dust grains interact eagerly with the local radiation field, and re-emit absorbed photons at infrared wavelengths. Often infrared emission from dust grains is an important gauge towards understanding the properties of the entire state of the system. For these reasons, taking into account the dynamics of dust is paramount toward understanding to what degree dust changes the dynamics of astrophysical fluids (for example in fluid instabilities), and how strongly the dust distribution is anchored to the gas - and thus how good of a probe the observations of dust are for the unobserved gas. During my PhD, I have worked on tools and simulations that allow to answer these questions. I have extended the MPI-AMRVAC code by further developing the dust module, which now allows to simulate a mixture containing an arbitrary large number of dust fluids, each with different grains sizes and compositions. After extensive testing and benchmarking, the dust module has been applied to the investigation of the effect of dust on Kelvin-Helmholtz instabilities (KHI), an instability that is often encountered in astrophysical as well as terrestrial fluids. We find that dust slows and sometimes even stabilises the instability, and that the KHI can result in significant enhancements in dust density, which is highly important in models that require high dust densities, such as those relevant for early planet formation. In our investigation of the KHI we found interesting parallels between our 3D simulations and the structures observed in molecular clouds, such as the formation of filamentary structures around low density regions. In a further investigation, even more similarities between observations and simulations are revealed. The typical observed shape of the probability density function of molecular clouds can be reproduced by simulation, providing a possible explanation for the deviation from the log-normal profile that is seen for some molecular clouds. To allow direct comparison between simulations and observations, we have coupled our (dust) dynamic simulations with the SKIRT dust radiative transfer code. Doing so, we modelled the infrared observations of KH ripples in the Orion molecular cloud. Our results show that the observations are highly dependent on the relative orientation of the observer and the configuration of the objects in the observations, and that the combination of hydrodynamics and radiative transfer can provide further insight in the observations. In the last chapter, we have a look at dust-producing Wolf-Rayet stars. The fact that these massive stars even allow dust to survive in their vicinity came as a surprise, and now more than three decades after this has first been recognised this paradox has not been fully comprehended. We investigate the dust dynamics of the dust-producing binary WR98a in 2D and 3D, and have a look how several parameters influence the dusty pinwheel structures observed in the system.nrpages: 218status: publishe