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Monday, 4 November 2019

One of the mechanisms behind thermonuclear supernovae recreated in the laboratory


All stars do not end their lives in the same way. The most massive stars, once their thermonuclear reactions exhausted, explode into a phenomenon called supernova. Supernovas can be classified into two types: thermonuclear supernovae (type Ia) and collapsing supernovas (type II, Ib, Ic). One of the possible mechanisms behind type Ia supernovas is the deflagration-detonation transition (DDT). Recently, physicists have developed a model of experimentally validated DDT, allowing a better understanding of this mechanism, which could also allow to develop new means of propulsion.

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The nature of type Ia supernovae (SNIa), the thermonuclear supernovae, is still very little constrained. There is a general consensus that SNIa explosions are caused by rapid thermonuclear combustion of stars with a mass near or below the Chandrasekhar mass limit (1.4 solar masses). Beyond these models, however, the exact mechanisms of the SNIa remain unclear, with a number of possible scenarios.

In some forms of supernovae and chemical explosions, a flame moving at subsonic velocities (deflagration) spontaneously evolves into supersonic shock (detonation) -speed, greatly increasing power. The mechanism of this deflagration-detonation transition (DDT) is still poorly understood.

Recreate the detonation mechanism of supernovae in the laboratory

A team of physicists has developed a unified theory of the turbulence-induced DDT mechanism, which describes the mechanism and conditions of initiation of detonation during unconfined chemical and thermonuclear explosions. The model was validated using experiments with chemical flames and numerical simulations of thermonuclear flames. The study was published in the journal Science.

Physicists have recreated the detonation-detonation mechanism in which a flame encounters a shock wave, resulting in a sudden detonation. Credits: Alexei Y. Poludnenko et al. 2019

" When we started deepening the question, we discovered that we can actually achieve this mechanism between a passive flame and a flame that becomes very active, " says Ahmed. " It reaches a point producing a detonation, which is essentially a supernova ."

The death of stars 10 to 100 times larger than the Sun generates an explosion called supernova. In a galaxy like the Milky Way, a supernova occurs every 50 years or so. But the mechanism underlying the initiation of the supernova is still little understood by astrophysicists.

The mechanism of the explosion-detonation transition experimentally validated

" How do the flames accelerate spontaneously and turn into detonations? We know that the detonations exist, but the question was: what is the missing link explaining how the star goes from a combustion mode to a detonation mode so brutally? Ahmed explains.

The mechanism explored by Ahmed's team takes a passive flame, like that of a candle, and turns it into an energetic flame. " When we started to notice that the flames could accelerate, we had to develop this unique facility to explore the phenomenon,  " says Ahmed.

The researchers first modeled the DDT mechanism using plasma and then experimentally confirmed. Credits: Alexei Y. Poludnenko et al. 2019

The installation was in the form of a 5 x 5 cm shock tube, which induces turbulence and allows the passive flame to interact with it until it propels itself until a detonation.

This study will not only provide a better understanding of the explosion-detonation mechanism of Type Ia supernovae, but also the development of hypersonic propulsion means on Earth.

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