Banda-Barragan, Wladimir Eduardo
Description
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...[Show more] 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.
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