580 research outputs found
Surface detonation in type Ia supernova explosions?
We explore the evolution of thermonuclear supernova explosions when the
progenitor white dwarf star ignites asymmetrically off-center. Several
numerical simulations are carried out in two and three dimensions to test the
consequences of different initial flame configurations such as spherical
bubbles displaced from the center, more complex deformed configurations, and
teardrop-shaped ignitions. The burning bubbles float towards the surface while
releasing energy due to the nuclear reactions. If the energy release is too
small to gravitationally unbind the star, the ash sweeps around it, once the
burning bubble approaches the surface. Collisions in the fuel on the opposite
side increase its temperature and density and may -- in some cases -- initiate
a detonation wave which will then propagate inward burning the core of the star
and leading to a strong explosion. However, for initial setups in two
dimensions that seem realistic from pre-ignition evolution, as well as for all
three-dimensional simulations the collimation of the surface material is found
to be too weak to trigger a detonation.Comment: 5 pages, 3 figures, in: Proceedings of the SciDAC 2006 Meeting,
Denver June 25-26 2006, also available at
http://herald.iop.org/jpcs46/m51/gbr//link/40
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
Modeling magnetohydrodynamics and non equilibrium SoHO/UVCS line emission of CME shocks
We provide a guideline to interpret the UVCS emission lines (in particular O
VI and Si XII) during shock wave propagation in the outer solar corona. We use
a numerical MHD model performing a set of simulations of shock waves generated
in the corona and from the result we compute the plasma emission for the O VI
and Si XII including the effects of NEI. We analyze the radiative and spectral
properties of our model with the support of a detailed radiation model
including Doppler dimming and an analytical model for shocks, and, finally, we
synthesize the expected O VI 1032A line profile. We explain several spectral
features of the observations like the absence of discontinuities in the O VI
emission during the shock passage, the brightening of Si XII emission and the
width of the lines. We use our model also to give very simple and general
predictions for the strength of the line wings due to the ions shock heating
and on the line shape for Limb CMEs or Halo CMEs. The emission coming from
post-shock region in the solar corona roughly agrees with the emission from a
simple planar and adiabatic shock, but the effect of thermal conduction and the
magnetic field may be important depending on the event parameters. Doppler
dimming significantly influences the O VI emission while Si XII line brightens
mainly because of the shock compression. Significant shock heating is
responsible for the wide and faint component of the O VI line usually observed
which may be taken as a shock signature in the solar corona.Comment: 11 pages, 12 figures, 2 appendixe
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
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
A New MHD Code with Adaptive Mesh Refinement and Parallelization for Astrophysics
A new code, named MAP, is written in Fortran language for
magnetohydrodynamics (MHD) calculation with the adaptive mesh refinement (AMR)
and Message Passing Interface (MPI) parallelization. There are several optional
numerical schemes for computing the MHD part, namely, modified Mac Cormack
Scheme (MMC), Lax-Friedrichs scheme (LF) and weighted essentially
non-oscillatory (WENO) scheme. All of them are second order, two-step,
component-wise schemes for hyperbolic conservative equations. The total
variation diminishing (TVD) limiters and approximate Riemann solvers are also
equipped. A high resolution can be achieved by the hierarchical
block-structured AMR mesh. We use the extended generalized Lagrange multiplier
(EGLM) MHD equations to reduce the non-divergence free error produced by the
scheme in the magnetic induction equation. The numerical algorithms for the
non-ideal terms, e.g., the resistivity and the thermal conduction, are also
equipped in the MAP code. The details of the AMR and MPI algorithms are
described in the paper.Comment: 44 pages, 16 figure
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
Evaluating Systematic Dependencies of Type Ia Supernovae: The Influence of Deflagration to Detonation Density
We explore the effects of the deflagration to detonation transition (DDT)
density on the production of Ni-56 in thermonuclear supernova explosions (type
Ia supernovae). Within the DDT paradigm, the transition density sets the amount
of expansion during the deflagration phase of the explosion and therefore the
amount of nuclear statistical equilibrium (NSE) material produced. We employ a
theoretical framework for a well-controlled statistical study of
two-dimensional simulations of thermonuclear supernovae with randomized initial
conditions that can, with a particular choice of transition density, produce a
similar average and range of Ni-56 masses to those inferred from observations.
Within this framework, we utilize a more realistic "simmered" white dwarf
progenitor model with a flame model and energetics scheme to calculate the
amount of Ni-56 and NSE material synthesized for a suite of simulated
explosions in which the transition density is varied in the range 1-3x10^7
g/cc. We find a quadratic dependence of the NSE yield on the log of the
transition density, which is determined by the competition between plume rise
and stellar expansion. By considering the effect of metallicity on the
transition density, we find the NSE yield decreases by 0.055 +/- 0.004 solar
masses for a 1 solar metallicity increase evaluated about solar metallicity.
For the same change in metallicity, this result translates to a 0.067 +/- 0.004
solar mass decrease in the Ni-56 yield, slightly stronger than that due to the
variation in electron fraction from the initial composition. Observations
testing the dependence of the yield on metallicity remain somewhat ambiguous,
but the dependence we find is comparable to that inferred from some studies.Comment: 15 pages, 13 figures, accepted to ApJ on July 6, 201
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