493 research outputs found
Multi-dimensional numerical simulations of type Ia supernova explosions
The major role type Ia supernovae play in many fields of astrophysics and in
particular in cosmological distance determinations calls for self-consistent
models of these events. Since their mechanism is believed to crucially depend
on phenomena that are inherently three-dimensional, self-consistent numerical
models of type Ia supernovae must be multi-dimensional. This field has recently
seen a rapid development, which is reviewed in this article. The different
modeling approaches are discussed and as an illustration a particular explosion
model -- the deflagration model -- in a specific numerical implementation is
presented in greater detail. On this exemplary case, the procedure of
validating the model on the basis of comparison with observations is discussed
as well as its application to study questions arising from type Ia supernova
cosmology.Comment: 30 pages, 7 figures (Fig. 6 with reduced resolution
Flame-driven deflagration-to-detonation transitions in Type Ia supernovae?
Although delayed detonation models of thermonuclear explosions of white
dwarfs seem promising for reproducing Type Ia supernovae, the transition of the
flame propagation mode from subsonic deflagration to supersonic detonation
remains hypothetical. A potential instant for this transition to occur is the
onset of the distributed burning regime, i.e. the moment when turbulence first
affects the internal flame structure. Some studies of the burning microphysics
indicate that a deflagration-to-detonation transition may be possible here,
provided the turbulent intensities are strong enough. Consequently, the
magnitude of turbulent velocity fluctuations generated by the deflagration
flame is analyzed at the onset of the distributed burning regime in several
three-dimensional simulations of deflagrations in thermonuclear supernovae. It
is shown that the corresponding probability density functions fall off towards
high turbulent velocity fluctuations much more slowly than a Gaussian
distribution. Thus, values claimed to be necessary for triggering a detonation
are likely to be found in sufficiently large patches of the flame. Although the
microphysical evolution of the burning is not followed and a successful
deflagration-to-detonation transition cannot be guaranteed from simulations
presented here, the results still indicate that such events may be possible in
Type Ia supernova explosions.Comment: 6 pages, 2 figures, to appear in ApJ 668, 1103 (2007
Numerical simulations of multi-scale astrophysical problems: The example of Type Ia supernovae
Vastly different time and length scales are a common problem in numerical
simulations of astrophysical phenomena. Here, we present an approach to
numerical modeling of such objects on the example of Type Ia supernova
simulations. The evolution towards the explosion proceeds on much longer time
scales than the explosion process itself. The physical length scales relevant
in the explosion process cover 11 orders of magnitude and turbulent effects
dominate the physical mechanism. Despite these challenges, three-dimensional
simulations of Type Ia supernova explosions have recently become possible and
pave the way to a better understanding of these important astrophysical
objects.Comment: 10 pages, 1 figure; in "Modelling and Simulation in Science",
Proceedings of the 6th International Workshop on Data Analysis in Astronomy
"Livio Scarsi", Erice, Italy 15 - 22 April 2007 (World Scientific, 2008
Off-center ignition in type Ia supernova: I. Initial evolution and implications for delayed detonation
The explosion of a carbon-oxygen white dwarf as a Type Ia supernova is known
to be sensitive to the manner in which the burning is ignited. Studies of the
pre-supernova evolution suggest asymmetric, off-center ignition, and here we
explore its consequences in two- and three-dimensional simulations. Compared
with centrally ignited models, one-sided ignitions initially burn less and
release less energy. For the distributions of ignition points studied, ignition
within two hemispheres typically leads to the unbinding of the white dwarf,
while ignition within a small fraction of one hemisphere does not. We also
examine the spreading of the blast over the surface of the white dwarf that
occurs as the first plumes of burning erupt from the star. In particular, our
studies test whether the collision of strong compressional waves can trigger a
detonation on the far side of the star as has been suggested by Plewa et al.
(2004). The maximum temperature reached in these collisions is sensitive to how
much burning and expansion has already gone on, and to the dimensionality of
the calculation. Though detonations are sometimes observed in 2D models, none
ever happens in the corresponding 3D calculations. Collisions between the
expansion fronts of multiple bubbles also seem, in the usual case, unable to
ignite a detonation. "Gravitationally confined detonation" is therefore not a
robust mechanism for the explosion. Detonation may still be possible in these
models however, either following a pulsation or by spontaneous detonation if
the turbulent energy is high enough.Comment: 13 pages, 10 figures (resolution of some figures reduced to comply
with astro-ph file size restriction); submitted to the Astrophysical Journal
on 8/3/200
The Cellular Burning Regime in Type Ia Supernova Explosions - I. Flame Propagation into Quiescent Fuel
We present a numerical investigation of the cellular burning regime in Type
Ia supernova explosions. This regime holds at small scales (i.e. below the
Gibson scale), which are unresolved in large-scale Type Ia supernova
simulations. The fundamental effects that dominate the flame evolution here are
the Landau-Darrieus instability and its nonlinear stabilization, leading to a
stabilization of the flame in a cellular shape. The flame propagation into
quiescent fuel is investigated addressing the dependence of the simulation
results on the specific parameters of the numerical setup. Furthermore, we
investigate the flame stability at a range of fuel densities. This is directly
connected to the questions of active turbulent combustion (a mechanism of flame
destabilization and subsequent self-turbulization) and a
deflagration-to-detonation transition of the flame. In our simulations we find
no substantial destabilization of the flame when propagating into quiescent
fuels of densities down to ~10^7 g/cm^3, corroborating fundamental assumptions
of large-scale SN Ia explosion models. For these models, however, we suggest an
increased lower cutoff for the flame propagation velocity to take the cellular
burning regime into account.Comment: 12 pages, 2 tables, 10 figures, resolution of figures degraded due to
archive file size restrictions, submitted to A&
New numerical solver for flows at various Mach numbers
Many problems in stellar astrophysics feature flows at low Mach numbers.
Conventional compressible hydrodynamics schemes frequently used in the field
have been developed for the transonic regime and exhibit excessive numerical
dissipation for these flows. While schemes were proposed that solve
hydrodynamics strictly in the low Mach regime and thus restrict their
applicability, we aim at developing a scheme that correctly operates in a wide
range of Mach numbers. Based on an analysis of the asymptotic behavior of the
Euler equations in the low Mach limit we propose a novel scheme that is able to
maintain a low Mach number flow setup while retaining all effects of
compressibility. This is achieved by a suitable modification of the well-known
Roe solver. Numerical tests demonstrate the capability of this new scheme to
reproduce slow flow structures even in moderate numerical resolution. Our
scheme provides a promising approach to a consistent multidimensional
hydrodynamical treatment of astrophysical low Mach number problems such as
convection, instabilities, and mixing in stellar evolution.Comment: 16 pages, 8 figures, accepted for publication by A&
Thermonuclear Supernovae
The application of Type Ia supernovae (SNe Ia) as distance indicators in
cosmology calls for a sound understanding of these objects. Recent years have
seen a brisk development of astrophysical models which explain SNe Ia as
thermonuclear explosions of white dwarf stars. While the evolution of the
progenitor is still uncertain, the explosion mechanism certainly involves the
propagation of a thermonuclear flame through the white dwarf star.
Three-dimensional hydrodynamical simulations allowed to study a wide variety of
possibilities involving subsonic flame propagation (deflagrations), flames
accelerated by turbulence, and supersonic detonations. These possibilities lead
to a variety of scenarios. I review the currently discussed approaches and
present some recent results from simulations of the turbulent deflagration
model and the delayed detonation model.Comment: 25 pages, 7 figures (some with reduced resolution), invited review at
"Supernovae: lights in the darkness", October 3-5, 2007, Mao (Menorca), to
appear in Proceedings of Scienc
A Subgrid-scale Model for Deflagration-to-Detonation Transitions in Type Ia Supernova Explosion Simulations - Numerical implementation
A promising model for normal Type Ia supernova (SN Ia) explosions are delayed
detonations of Chandrasekhar-mass white dwarfs, in which the burning starts out
as a subsonic deflagration and turns at a later phase of the explosion into a
supersonic detonation. The mechanism of the underlying
deflagration-to-detonation transition (DDT) is unknown in detail, but necessary
conditions have been determined recently. The region of detonation initiation
cannot be spatially resolved in multi-dimensional full-star simulations of the
explosion. We develop a subgrid-scale (SGS) model for DDTs in thermonuclear
supernova simulations that is consistent with the currently known constraints.
The probability for a DDT to occur is calculated from the distribution of
turbulent velocities measured on the grid scale in the vicinity of the flame
and the fractal flame surface area that satisfies further physical constraints,
such as fuel fraction and fuel density. The implementation of our DDT criterion
provides a solid basis for simulations of thermonuclear supernova explosions in
the delayed detonation scenario. It accounts for the currently known necessary
conditions for the transition and avoids the inclusion of resolution-dependent
quantities in the model. The functionality of our DDT criterion is demonstrated
on the example of one three-dimensional thermonuclear supernova explosion
simulation.Comment: accepted for publication in Astronomy and Astrophysic
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