Magnetohydrodynamics of Wind-Cloud Interactions: Filament Formation in the Interstellar Medium

Abstract

Filaments are ubiquitous in the interstellar medium, yet their formation, internal structure, magnetic properties, and longevity have not been analysed in detail. In this thesis I report the results from a comprehensive numerical study that investigates the characteristics, formation, dynamics, and global evolution of filamentary structures arising from (magneto)hydrodynamic interactions between supersonic winds and interstellar clouds. Here I improve on previous wind-cloud simulations by utilising higher numerical resolutions, sharper density contrasts, more complex magnetic field configurations, and more realistic systems with turbulent clouds.

I use gas multi-tracking algorithms and state-of-the-art visualisation techniques to study the physical mechanisms acting upon wind-swept clouds. I find that material originally in the envelopes of the clouds is removed and transported downstream to form filamentary tails, while the cores of the clouds serve as footpoints and late-stage outer layers of these low-density tails. The evolution of filaments comprises four phases: 1) tail formation, 2) tail erosion, 3) footpoint dispersion, and 4) filament free floating. Overall, wind-cloud interactions produce filaments with aspect ratios >10, lateral expansions ~1-3 of the core radius, mixing fractions ~10-30%, velocity dispersions ~0.02-0.05 of the wind speed, and magnetic field amplifications by factors of ~10-100.

I find that the strength of magnetic fields regulates vorticity production: sinuous filamentary towers arise in non-magnetic environments, while strong magnetic fields inhibit small-scale Kelvin-Helmholtz perturbations at boundary layers making tails less turbulent. The orientation of magnetic fields also influences the morphology of filaments: magnetic field components aligned with the direction of the wind favour the formation of pressure-confined flux ropes inside the tails, whilst transverse components tend to form current sheets and favour the growth of Rayleigh-Taylor perturbations at the leading edge of the clouds.

I also investigate how turbulence influences the formation of filaments by sequentially adding log-normal density profiles, Gaussian velocity fields, and turbulent magnetic fields into the initial clouds. The porosity of turbulent density profiles aids the propagation of internal shocks through filament material, accelerating mixing and increasing the internal velocity dispersion. The inclusion of subsonically-turbulent velocity fields has little effect on the evolution, while supersonically-turbulent velocity fields accelerate the cloud expansion and subsequent break-up. Line stretching and compression amplify the magnetic energy of turbulent filaments creating highly-magnetised knots and sub-filaments along their tails. In all models the magnetic energy enhancement saturates when the ratio of turbulent kinetic to turbulent magnetic energy densities is ~5-10.

At the end of this thesis I discuss the relevance of this work for the study of clouds and filaments in the Galactic centre and provide my perspectives on potential future research in this field. Using ray-tracing techniques I create synthetic emission maps of wind-swept clouds and compare them with radio observations of high-latitude H I clouds and non-thermal filaments in this region of the Galaxy. I interpret these structures as remains of the interplay between outflows driven by localised star formation and dense clouds in the surrounding medium. The simulated morphology, lifespan, magnetic properties, and kinematics are consistent with those inferred from observations of these clouds and non-thermal filaments.