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Researchers at the Max Planck Institute for Astrophysics study
thermonuclear supernova explosions in three-dimensional simulations.
Type Ia supernovae (SNe Ia) play an important role in various
branches of astrophysics. They are one of the main sources of the
enrichment of the interstellar medium with heavy elements
(predominantly iron group elements) and thus have an impact on the
formation of stars and galaxies. They possess an exceptional
brightness - comparable to that of an entire galaxy consisting of
billions of stars. Moreover, SNe Ia are remarkably uniform in their
characteristics. For these reasons they were suggested as an ideal
tool for a geometrical surveying of the universe - using them as
"lighthouses" far away from our own galaxy. To reach the
accuracy required for this application, it is necessary to calibrate
the brightness of SNe Ia according to correlations of their
properties. These correlations, however, lack a theoretical
explanation so far (see also  current
research june 2000 and december
2002).
This motivates attempts to better understand the mechanism of SN Ia
explosions. One approach to this is to construct an astrophysical
model on the basis of general properties and to implement it into a
computer simulation. First successes of this method at the Max Planck
Institute for Astrophysics (MPA) were reported in an earlier research
highlight article. Progress in the numerical techniques as well
as increased computer power (the numerical models are very demanding
and can only be solved on massive parallel systems) facilitate more
detailed studies of the SN Ia model. Initial parameters of the models
playing an important role in the calibration mentioned above could be
explored in the first systematic studies based on three-dimensional
simulations performed by researchers at the Max Planck Institute for
Astrophysics. Simulations recently performed at that institute
comprise the full star (in contrast to only one spatial octant in
earlier simulations). This enables the investigation of asymmetry
effects. We will present such a model here.
The model favored in astrophysics explains Type Ia supernovae with
thermonuclear explosions of white dwarf stars. They are composed of
carbon and oxygen. A single white dwarf star is an inert
object. However, in the SN Ia model it accretes matter from a binary
companion until it reaches densities and temperatures in its center to
fuse the carbon and oxygen to heavier elements. A flame forms,
i.e. the fusion reaction proceeds in a tiny volume, most likely at the
surface of bubbles filled with burnt material. Such an initial flame
is shown in Fig. 1.
Due to heat conduction this flame burns from the center of the white
dwarf star outward. This proceeds with velocities lower than the local
sound speed and is termed deflagration. Some SN Ia models assume a
transition of this flame propagation mode to a supersonic detonation
driven by shock waves at later times. However, no physical mechanism
triggering this transition is known yet. The models discussed here are
based on the pure deflagration model.
A deflagration flame burning from the center of the white dwarf star
outward leaves hot and light burnt material behind. The fuel in front
of it is, however, cold and dense. This results in a density
stratification inverse to the gravitational field of the star, which
is therefore unstable. Thus, blobs of burning material form and ascend
into the fuel (see Fig. 2). At their interfaces shear flows
emerge. These effects lead to strong swirls. The resulting turbulent
motions deform the flame and thus enlarge its surface. This increases
the net burning rate of the flame and leads to the energetic explosion
(cf. Fig. 3).
Fig. 4 shows a snapshot at a time where burning has already
ceased. Large parts of the star are burnt in the explosion and expand
strongly. The configuration has lost its initial symmetric shape. For
comparison of the scales the white dwarf star at the stage of Fig. 1
is shown again in the lower left corner of Fig. 4.
The evolution and propagation of the flame front is visualized in the
movie. The color indicates a measure of the flame propagation
velocity.
The SN Ia model presented here is the first simulation comprising the
full star and leading to an explosion strength and an amount of burnt
material that come very close to the observed values. Details of such
numerical models will be analyzed in detail in future
investigations. It is planned to assess the models on the basis of
synthetic light curves and spectra which can be directly compared with
observations.
Friedrich Röpke and Wolfgang Hillebrandt
Literature:
F. K. Röpke und W. Hillebrandt (2004), submitted to
Astron. Astrophys. (preprint: astro-ph/0409286)
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