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  Current Research Highlight :: October 2004 all highlights

Three-dimensional simulations of Type Ia supernova explosions

Fig. 1: start of the simulation

Fig. 2: 0.3 seconds

Fig. 3: 0.6 seconds

Fig. 4: 2 seconds

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 linkPfeil.gifcurrent research june 2000 and linkPfeil.gifdecember 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 linkPfeil.gifresearch 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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