520 research outputs found
The Evolution of Adiabatic Supernova Remnants in a Turbulent, Magnetized Medium
(Abridged) We present the results of three dimensional calculations for the
MHD evolution of an adiabatic supernova remnant in both a uniform and turbulent
interstellar medium using the RIEMANN framework of Balsara. In the uniform
case, which contains an initially uniform magnetic field, the density structure
of the shell remains largely spherical, while the magnetic pressure and
synchrotron emissivity are enhanced along the plane perpendicular to the field
direction. This produces a bilateral or barrel-type morphology in synchrotron
emission for certain viewing angles. We then consider a case with a turbulent
external medium as in Balsara & Pouquet, characterized by .
Several important changes are found. First, despite the presence of a uniform
field, the overall synchrotron emissivity becomes approximately spherically
symmetric, on the whole, but is extremely patchy and time-variable, with
flickering on the order of a few computational time steps. We suggest that the
time and spatial variability of emission in early phase SNR evolution provides
information on the turbulent medium surrounding the remnant. The
shock-turbulence interaction is also shown to be a strong source of
helicity-generation and, therefore, has important consequences for magnetic
field generation. We compare our calculations to the Sedov-phase evolution, and
discuss how the emission characteristics of SNR may provide a diagnostic on the
nature of turbulence in the pre-supernova environment.Comment: ApJ, in press, 5 color figure
A turbulent MHD model for molecular clouds and a new method of accretion on to star-forming cores
We describe the results of a sequence of simulations of gravitational
collapse in a turbulent magnetized region. The parameters are chosen to be
representative of molecular cloud material. We find that several protostellar
cores and filamentary structures of higher than average density form. The
filaments inter-connect the high density cores. Furthermore, the magnetic field
strengths are found to correlate positively with the density, in agreement with
recent observations. We make synthetic channel maps of the simulations and show
that material accreting onto the cores is channelled along the magnetized
filamentary structures. This is compared with recent observations of S106, and
shown to be consistent with these data. We postulate that this mechanism of
accretion along filaments may provide a means for molecular cloud cores to grow
to the point where they become gravitationally unstable and collapse to form
stars.Comment: Accepted by MNRA
Dust Settling in Magnetorotationally-Driven Turbulent Discs II: The Pervasiveness of the Streaming Instability and its Consequences
We present a series of simulations of turbulent stratified protostellar discs
with the goal of characterizing the settling of dust throughout a minimum-mass
solar nebula. We compare the evolution of both compact spherical grains, as
well as highly fractal grains. Our simulations use a shearing-box formulation
to study the evolution of dust grains locally within the disc, and collectively
our simulations span the entire extent of a typical accretion disc. The dust is
stirred by gas that undergoes MRI-driven turbulence. This establishes a steady
state scale height for the dust that is different for dust of different sizes.
This sedimentation of dust is an important first step in planet formation and
we predict that ALMA should be able to observationally verify its existence.
When significant sedimentation occurs, the dust will participate in a streaming
instability that significantly enhances the dust density. We show that the
streaming instability is pervasive in the outer disc. We characterize the scale
heights of dust whose size ranges from a few microns to a few centimeters. We
find that for spherical grains, a power-law relationship develops for the scale
height with grain size, with a slope that is slightly steeper than -1/2. The
sedimentation is strongest in the outer disc and increases for large grains.
The results presented here show that direct measurements of grain settling can
be made by ALMA and we present favorable conditions for observability. The
streaming instability should also be directly observable and we provide
conditions for directly observing it. We calculate collision rates and growth
rates for the dust grains in our simulations of various sizes colliding with
other grains, and find that these rates are significantly enhanced through the
density enhancement arising from the streaming instability.Comment: 39 pages, 13 figures, submitted to MNRAS. Abstract is abridge
Second Order Accurate Schemes for Magnetohydrodynamics With Divergence-Free Reconstruction
In this paper we study the problem of divergence-free numerical MHD and show
that the work done so far still has four key unresolved issues. We resolve
those issues in this paper. The problem of reconstructing MHD flow variables
with spatially second order accuracy is also studied. The other goal of this
paper is to show that the same well-designed second order accurate schemes can
be formulated for more complex geometries such as cylindrical and spherical
geometry. Being able to do divergence-free reconstruction in those geometries
also resolves the problem of doing AMR in those geometries. The resulting MHD
scheme has been implemented in Balsara's RIEMANN framework for parallel,
self-adaptive computational astrophysics. The present work also shows that
divergence-free reconstruction and the divergence-free time-update can be done
for numerical MHD on unstructured meshes. All the schemes designed here are
shown to be second order accurate. Several stringent test problems are
presented to show that the methods work, including problems involving high
velocity flows in low plasma-b magnetospheric environments.Comment: 85 pages, 6 figure
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