1,692 research outputs found
Delayed Detonation at a Single Point in Exploding White Dwarfs
Delayed detonation in an exploding white dwarf, which propagates from an
off-center transition point, rather than from a spherical transition shell, is
described and simulated. The differences between the results of 2D simulations
and the 1D case are presented and discussed. The two dimensional effects become
significant in transition density below 3.e7 g/cm^3, where the energetics, the
production of Fe group elements and the symmetry of the explosion are all
affected. In the 2D case the explosion is less energetic and less Ni is
produced in the detonation phase of the explosion. For low transition density
the reduction in Ni mass can reach 20-30 percent. The asymmetry in abundances
between regions close to the transition point and regions far from that point
is large, and could be a source to polarization patterns in the emitted light.
We conclude that the spatial and temporal distribution of transition locations,
is an important parameter which must be included in delayed detonation models
for Type Ia supernovae. \Comment: 11 pages, 1 figur
The Thermonuclear Explosion Of Chandrasekhar Mass White Dwarfs
The flame born in the deep interior of a white dwarf that becomes a Type Ia
supernova is subject to several instabilities. We briefly review these
instabilities and the corresponding flame acceleration. We discuss the
conditions necessary for each of the currently proposed explosion mechanisms
and the attendant uncertainties. A grid of critical masses for detonation in
the range - g cm is calculated and its
sensitivity to composition explored. Prompt detonations are physically
improbable and appear unlikely on observational grounds. Simple deflagrations
require some means of boosting the flame speed beyond what currently exists in
the literature. ``Active turbulent combustion'' and multi-point ignition are
presented as two plausible ways of doing this. A deflagration that moves at the
``Sharp-Wheeler'' speed, , is calculated in one dimension
and shows that a healthy explosion is possible in a simple deflagration if the
front moves with the speed of the fastest floating bubbles. The relevance of
the transition to the ``distributed burning regime'' is discussed for delayed
detonations. No model emerges without difficulties, but detonation in the
distributed regime is plausible, will produce intermediate mass elements, and
warrants further study.Comment: 28 pages, 4 figures included, uses aaspp4.sty. Submitted to Ap
Detonating Failed Deflagration Model of Thermonuclear Supernovae I. Explosion Dynamics
We present a detonating failed deflagration model of Type Ia supernovae. In
this model, the thermonuclear explosion of a massive white dwarf follows an
off-center deflagration. We conduct a survey of asymmetric ignition
configurations initiated at various distances from the stellar center. In all
cases studied, we find that only a small amount of stellar fuel is consumed
during deflagration phase, no explosion is obtained, and the released energy is
mostly wasted on expanding the progenitor. Products of the failed deflagration
quickly reach the stellar surface, polluting and strongly disturbing it. These
disturbances eventually evolve into small and isolated shock-dominated regions
which are rich in fuel. We consider these regions as seeds capable of forming
self-sustained detonations that, ultimately, result in the thermonuclear
supernova explosion. Preliminary nucleosynthesis results indicate the model
supernova ejecta are typically composed of about 0.1-0.25 Msun of silicon group
elements, 0.9-1.2 Msun of iron group elements, and are essentially carbon-free.
The ejecta have a composite morphology, are chemically stratified, and display
a modest amount of intrinsic asymmetry. The innermost layers are slightly
egg-shaped with the axis ratio ~1.2-1.3 and dominated by the products of
silicon burning. This central region is surrounded by a shell of silicon-group
elements. The outermost layers of ejecta are highly inhomogeneous and contain
products of incomplete oxygen burning with only small admixture of unburned
stellar material. The explosion energies are ~1.3-1.5 10^51 erg.Comment: ApJ in press; 21 pages, 36 figures at reduced resolution; high
resolution version available at
http://flash.uchicago.edu/~tomek/Papers/DFD_I_r2.pd
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