Nowadays astrophysicists have quite a clear picture of the processes occurring in supernova explosions. However, for quite some years scientists have been puzzling about exactly how the transport of energy during a "Star Bang" happens. In a current research project scientists from the Excellence Cluster Universe have gained new insight into this question. Close supernovae are rare and hardly ever observed "live". Therefore, Andreas Marek and Hans-Thomas Janka, two researchers from the Max-Planck-Institute for Astrophysics, simulated supernova processes in a computer model. For the first time researchers succeeded in reproducing the interactions of neutrinos and matter within stars of 11 to 15 times the mass of the sun - a project which so far has used up more than ten million hours on several supercomputers. The results will be published in one of the next issues of Astrophysical Journal.

Stars are the chemical factories of the universe. In the core of the star, hydrogen atoms fuse to helium under inconceivable pressure and extremely high temperatures. Under suitable conditions the reaction chain continues: In other words, from the fusion of helium atoms, the heavier element carbon is produced, which in turn produces oxygen. Stars of great mass, whose mass is at least eight times the mass of our sun, continue the combustion processes in the core to produce even heavier elements; stars with more than ten times the mass of the sun even continue to produce iron. By contrast, the end of heavy stars is disproportionately dramatic compared to their light colleagues - after a lifespan of 100 million years at the most, they end their existence with a tremendous supernova explosion.

During a supernova, it is presumed that massive stars at first implode. In their advanced state, stars resemble an onion: In the center there is a firm iron core with surrounding layers containing the lighter elements up to hydrogen. With the production of iron, the combustion processes responsible for the star's force equilibrium stop: In order to merge iron nuclei it would be necessary to apply energy from outside. Thus, the star becomes the victim of gravitation and collapses. During this process, gravity compresses the core more and more until the structure of the iron nuclei starts to dissolve: The electrons fuse with the protons and as a result a neutron star and a large amount of neutrinos emerge.

With the star's collapse, the matter of the star's outer layers crashes onto the central neutron core. Because of the collision onto the compact core, a shock front is formed and is reflected towards the outer shells of the collapsing star. The neutron star's intensive neutrino flow heats the matter behind the shock wave causing a major expansion. Thus, the outer layers of the star are flung away and the star bursts apart in a gigantic supernova explosion. The final relic is a small neutron star with a diameter of approximately 20 kilometers or in very rare cases a Black Hole.

As plausible as this model sounds - it only applies to stars with no more than ten times the mass of the sun. In the case of heavier stars the explanation has a flaw: According to computer calculations the neutrino-triggered explosion comes to a halt after approximately 100 kilometers. The reason for this is the dense material within the core that decelerates the neutrinos. Moreover, in the early phase of the supernova, debris from the outer layer falls into the center and interferes with the dispersion of the shock wave. However, observations of supernovae and supernova-relics show that the shock front at a radius of 100 million kilometers finally reaches the surface of the star to demolish the star's outer layer. Therefore, it becomes clear that the explosion needs a second attempt. But what happens and what generates the necessary energy?

With their simulations of stars with 11 to 15 times the mass of the sun, scientists confirmed an assumption that had been discussed in the literature for quite a while. The simulation suggested that even the explosion of such massive stars could be powered by neutrinos. However, in contrast to smaller stars, for massive stars the crucial impetus is given by hydrodynamic instabilities. The star's layers, heated by neutrinos, are swirled by convective currents, similar to porridge boiling in a pot. During this process, the matter develops mushroom-shaped bubbles, in which hot plasma arises. However, the decisive trigger is a phenomenon called "Standing Accretion Shock Instability" (SASI) that was not taken into consideration in earlier models. This phenomenon causes the shock front to oscillate in growing amplitudes and to "bulge" more and more. Thus, the shock wave will be pushed to higher distances and the convection will increase. Consequently, a third effect applies: in the SASI-model, the matter is exposed to the high energy neutrinos much longer allowing for a clearly higher transfer of energy.

"Our tests on two-dimensional computer models represent an important step forward in understanding how stars of a high mass, with more than ten times the mass of the sun, explode", explains Hans-Thomas Janka. "Maybe there are still other phenomena which intensify the explosion caused by neutrinos and hydrodynamic instabilities. A group of competitors claim, for example, that SASI could cause great pulsation and oscillation of the young neutron star, which then would generate sound waves like a bell. The energy of these sound waves could also contribute towards getting the explosion started. For this reason, in the future we will be concentrating on combined effect mechanisms in our simulation calculations."

Janka remarks that the current successful simulations are an important piece in the puzzle. However, for the big picture there are quite some pieces missing. "We will still need a couple of years before we solve the problem of supernova explosions. Furthermore we still have to transfer our 2D-simulations into a three-dimensional computer model. The physics of neutrino-triggered energy transport are so complex, that 3D-simulations push even high-performance computers to their limits".

Barbara Wankerl (Public Outreach Coordinator, Exzellenzcluster `Universe')

Publication

A. Marek und H.-Thomas Janka, "Delayed neutrino-driven supernova explosions aided by the standing accretion-shock instability.", The Astrophysical Journal, in the press http://xxx.uni-augsburg.de/abs/0708.3372

Further Information

Press Release Cluster of Excellence `Universe'

Acknowledgement:

For further information please contact:

Dr. Hans-Thomas Janka
Max-Planck-Institut für Astrophysik, Garching
Tel.: +49 89 30000-2228
email: thjmpa-garching.mpg.de

Barbara Wankerl
Excellence Cluster Universe
Technische Universität München
Tel.: +49 89 35831-7105
email: barbara.wankerluniverse-cluster.de