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Supernova Simulations Still Defy Explosions

Researchers at the Max-Planck-Institute for Astrophysics have performed the worldwide most advanced computer simulations of the gravitational collapse of massive, evolved stars. Although the crucial interactions of elementary particles were followed with unprecedented accuracy, the models fail to produce the expected supernova explosions.

Simulation einer Supernova

Figure 1: Four snapshots during the simulation of the collapse of a rotating star with 15 solar masses. The pictures show violent (convective) mass motions in the region of neutrino heating around the neutron star at the center. Between 0.18 seconds (upper left) and about 0.26 seconds (lower right) after the formation of the neutron star, the supernova shock front (which is visible as the discontinuity between blue and green) reveals huge pulsations and strong deformation. The displayed region is 620 km wide, the rotation axis goes vertically through the center (MPEG movie (4.4M)).

Stossradius als Funktion der Zeit

Figure 2: Temporal variations of the mean shock radius for simulations of three stars with different masses (from top to bottom: 11.2, 15, and 20 solar masses). In none of the cases the shock expands farther than 250 kilometers, no explosion occurs. The thin lines display the results of spherically symmetric calculations compared to the two-dimensional runs, which demonstrate the important role of violent convective mass motions.

Stars with more than about ten times the mass of our Sun develop iron cores at their center which eventually become unstable and collapse to neutron stars. The interior of neutron stars is denser than nuclear matter and initially extremely hot. Particle reactions at such conditions create neutrinos in huge numbers. These elementary particles have no electric charge and less than one millionth of the electron mass. They eventually escape from the dense neutron star, but before leaving the stellar interior they deposit some of their energy in the still infalling outer layers of the star. This neutrino heating is believed to cause the violent disruption of the star in a supernova explosion. Such catastrophic stellar deaths are not only among the brightest events in the universe and the birth places of neutron stars or black holes, they are also the sources of elements like iron, silicon, and oxygen, which are indispensable to form rocky planets and human life. Understanding the mechanism of the explosion is therefore of fundamental importance for a broad variety of astrophysical problems. (see "How do massive stars explode?")

A group of researchers at the Max-Planck-Institute for Astrophysics set out to test the current theory with so far unattained precision. Running a newly developed computer program on Germany's fastest supercomputer, the IBM "Regatta" system at the Rechenzentrum of the Max-Planck-Society in Garching, the group for the first time achieved to describe the neutrino production and interactions in great detail. The calculations required each a total of several hundred thousand trillion (several 10^17) floating point operations, which ranks them among the most expensive computer simulations ever done. They also included the effects of stellar rotation and of violent anisotropic plasma motions. Convective processes in the supernova core had been recognized to accelerate the energy transport inside the neutron star and to enhance the deposition of energy by neutrinos in the outer stellar layers, thus supporting the explosion of the star.

The outcome of these worldwide most elaborate supernova simulations was disappointing: No explosions could be obtained. This negative result shatters the widely accepted view of how the explosion starts. Theorists have to reconsider their ideas. What is missing in the current models? Do we understand the properties of the nuclear medium in the neutron star well enough? Do we really know how neutrinos interact with particles in the very dense plasma? Are three-dimensional effects important and do the current two-dimensional simulations therefore miss crucial physics? Can magnetic fields be safely ignored as in the models so far?

The latest generation of accurate supernova models therefore sheds new light on these open questions of supernova theory. They will keep astrophysicists, nuclear physicists and particle physicists busy for many years to come.

Robert Buras, Konstantinos Kifonidis, Markus Rampp
and Hans-Thomas Janka

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