101 research outputs found
Draping of Cluster Magnetic Fields over Bullets and Bubbles -- Morphology and Dynamic Effects
High-resolution X-ray observations have revealed cavities and `cold fronts'
with sharp edges in temperature, density, and metallicity within galaxy
clusters. Their presence poses a puzzle since these features are not expected
to be hydrodynamically stable, or to remain sharp in the presence of diffusion.
However, a moving core or bubble in even a very weakly magnetized plasma
necessarily sweeps up enough magnetic field to build up a dynamically important
sheath around the object; the layer's strength is set by a competition between
`plowing up' of field and field lines slipping around the core. We show that a
two-dimensional approach to the problem is quite generally not possible. In
three dimensions, we show with analytic arguments and in numerical experiments,
that this magnetic layer modifies the dynamics of a plunging core, greatly
modifies the effects of hydrodynamic instabilities on the core, modifies the
geometry of stripped material, and even slows the fall of the core through
magnetic tension. We derive an expression for the maximum magnetic field
strength, the thickness of the layer, and the opening angle of the magnetic
wake. The morphology of the magnetic draping layer implies the suppression of
thermal conduction across the layer, thus conserving strong temperature
gradients over the contact surface. The intermittent amplification of the
magnetic field as well as the injection of MHD turbulence in the wake of the
core is identified to be due to vorticity generation within the magnetic
draping layer. These results have important consequences for understanding the
physical properties and the complex gasdynamical processes of the intra-cluster
medium, and apply quite generally to motions through other magnetized
environments, e.g., the ISM.Comment: For version of this paper with interactive 3D graphics and
full-resolution figures, see http://www.cita.utoronto.ca/~ljdursi/draping/ .
19p, 26 figures, emulateapj format. Version accepted by ApJ - new references,
improved figure
Propagation of the First Flames in Type Ia Supernovae
We consider the competition of the different physical processes that can
affect the evolution of a flame bubble in a Type Ia supernovae -- burning,
turbulence and buoyancy. Even in the vigorously turbulent conditions of a
convecting white dwarf, thermonuclear burning that begins at a point near the
center (within 100 km) of the star is dominated by the spherical laminar
expansion of the flame, until the burning region reaches kilometers in size.
Consequently flames that ignite in the inner ~20 km promptly burn through the
center, and flame bubbles anywhere must grow quite large--indeed, resolvable by
large-scale simulations of the global system--for significant motion or
deformation occur. As a result, any hot-spot that successfully ignites into a
flame can burn a significant amount of white dwarf material. This potentially
increases the stochastic nature of the explosion compared to a scenario where a
simmering progenitor can have small early hot-spots float harmlessly away.
Further, the size where the laminar flame speed dominates other relevant
velocities sets a characteristic scale for fragmentation of larger flame
structures, as nothing--by definition--can easily break the burning region into
smaller volumes. This makes possible the development of semi-analytic
descriptions of the earliest phase of the propagation of burning in a Type Ia
supernovae, which we present here. Our analysis is supported by fully resolved
numerical simulations of flame bubbles.Comment: 33 pages, 14 figures, accepted for publication in Ap
Simulations of Astrophysical Fluid Instabilities
We present direct numerical simulations of mixing at Rayleigh-Taylor unstable
interfaces performed with the FLASH code, developed at the ASCI/Alliances
Center for Astrophysical Thermonuclear Flashes at the University of Chicago. We
present initial results of single-mode studies in two and three dimensions. Our
results indicate that three-dimensional instabilities grow significantly faster
than two-dimensional instabilities and that grid resolution can have a
significant effect on instability growth rates. We also find that unphysical
diffusive mixing occurs at the fluid interface, particularly in poorly resolved
simulations.Comment: 3 pages, 1 figure. To appear in the proceedings of the 20th Texas
Symposium on Relativistic Astrophysic
Large-Scale Simulations of Clusters of Galaxies
We discuss some of the computational challenges encountered in simulating the
evolution of clusters of galaxies. Eulerian adaptive mesh refinement (AMR)
techniques can successfully address these challenges but are currently being
used by only a few groups. We describe our publicly available AMR code, FLASH,
which uses an object-oriented framework to manage its AMR library, physics
modules, and automated verification. We outline the development of the FLASH
framework to include collisionless particles, permitting it to be used for
cluster simulation.Comment: 3 pages, 3 figures, to appear in Proceedings of the VII International
Workshop on Advanced Computing and Analysis Techniques in Physics Research
(ACAT 2000), Fermilab, Oct. 16-20, 200
Onset of Convectionon a Pre-Runaway White Dwarf
Observed novae abundances and explosion energies estimated from observations
indicate that there must be significant mixing of the heavier material of the
white dwarf (C+O) into the lighter accreted material (H+He). Accordingly, nova
models must incorporate a mechanism that will dredge up the heavier white dwarf
material, and fluid motions from an early convection phase is one proposed
mechanism.
We present results from two-dimensional simulations of classical nova
precursor models that demonstrate the beginning of a convective phase during
the `simmering' of a Nova precursor. We use a new hydrostatic equilibrium
hydrodynamics module recently developed for the adaptive-mesh code FLASH. The
two-dimensional models are based on the one-dimensional models of Ami Glasner,
and were evolved with FLASH from a pre-convective state to the onset of
convection.Comment: 5 pages, 4 figures, from the 2002 International Conference on
Classical Novae in Sitges, Spai
Mixing by Non-linear Gravity Wave Breaking on a White Dwarf Surface
We present the results of a simulation of a wind-driven non-linear gravity
wave breaking on the surface of a white dwarf. The ``wind'' consists of H/He
from an accreted envelope, and the simulation demonstrates that this breaking
wave mechanism can produce a well-mixed layer of H/He with C/O from the white
dwarf above the surface. Material from this mixed layer may then be transported
throughout the accreted envelope by convection, which would enrich the C/O
abundance of the envelope as is expected from observations of novae.Comment: 5 pages, 3 figures, to appear in the proceedings of the International
Conference on Classical Nova Explosions, Sitges, Spain, 20-24 May 200
Adaptively refined large eddy simulations of clusters
We present a numerical scheme for modelling unresolved turbulence in
cosmological adaptive mesh refinement codes. As a first application, we study
the evolution of turbulence in the intra-cluster medium and in the core of a
galaxy cluster. Simulations with and without subgrid scale model are compared
in detail. Since the flow in the ICM is subsonic, the global turbulent energy
contribution at the unresolved length scales is smaller than 1% of the internal
energy. We find that the production of turbulence is closely correlated with
merger events occurring in the cluster environment, and its dissipation locally
affects the cluster energy budget. Because of this additional source of
dissipation, the core temperature is larger and the density is smaller in the
presence of subgrid scale turbulence than in the standard adiabatic run,
resulting in a higher entropy core value.Comment: Submitted to ApJ, 14 pages, 14 figures, 3 table
Mixing by Non-linear Gravity Wave Breaking on a White Dwarf Surface
We present the results of a simulation of a wind-driven non-linear gravity
wave breaking on the surface of a white dwarf. The ``wind'' consists of H/He
from an accreted envelope, and the simulation demonstrates that this breaking
wave mechanism can produce a well-mixed layer of H/He with C/O from the white
dwarf above the surface. Material from this mixed layer may then be transported
throughout the accreted envelope by convection, which would enrich the C/O
abundance of the envelope as is expected from observations of novae.Comment: 5 pages, 3 figures, to appear in the proceedings of the International
Conference on Classical Nova Explosions, Sitges, Spain, 20-24 May 200
The Linear Instability of Astrophysical Flames in Magnetic Fields
Supernovae of Type Ia are used as standard candles for cosmological
observations despite the as yet incomplete understanding of their explosion
mechanism. In one model, these events are thought to result from subsonic
burning in the core of an accreting Carbon/Oxygen white dwarf that is
accelerated through flame wrinkling and flame instabilities. Many such white
dwarfs have significant magnetic fields. Here we derive the linear effects of
such magnetic fields on one flame instability, the well-known Landau-Darrieus
instability. When the magnetic field is strong enough that the flame is
everywhere sub-Alfvenic, the instability can be greatly suppressed.
Super-Alfvenic flames are much less affected by the field, with flames
propagating parallel to the field somewh at destabilized, and flames
propagating perpendicular to the field somewhat stabili zed. Trans-Alfvenic
parallel flames, however, like trans-Alfvenic parallel shocks, are seen to be
non-evolutionary; understanding the behavior of these flames will require
careful numerical simulation.Comment: 31 pp, 11 fig, submitted to Ap
- …