The evolution and nature of core-collapse-supernova explosions, pulsars, is a mystery, and one of the greatest unsolved problems in astrophysics. But a team led by Oak Ridge National Laboratory's Tony Mezzacappa is getting closer to explaining the origins of CCSN explosions with the help of Jaguar, a Cray XT5 supercomputer located at the Oak Ridge Leadership Computing Facility. This type of event is not observed in elliptical galaxies, but generally in the spiral arms of galaxies and in HII regions, where star formation is active. This support the idea that massive stars, with masses higher than 8 times the mass of the Sun, are the progenitors of this class of supernova.
In an effort to locate the source of the magnetic fields, the team simulated a supernova progenitor, or a star in its pre-supernova phase, using tens of millions of hours on Jaguar, the nation's fastest supercomputer.
Core-collapsed supernova (CCSN) remnants are commonly known as pulsars, and when it comes to magnetic fields, pulsars rank number one in the stellar community. These highly magnetized, rapidly rotating neutron stars get their name from the seemingly pulsing beam of light they emit, similar to the varying brightness produced by lighthouses as they rotate. This rotation is thought to be a big factor in determining the strength of a pulsar's magnetic field-the faster a star rotates the stronger its magnetic fields.
Supernova progenitors tend to be slower-rotating stars. Nevertheless, the simulations of these progenitors revealed a robust magnetic-field-generation mechanism, contradicting accepted theory that rotation could be a primary driver.
This discovery builds on the team's previous work, which together with the latest simulations reveals that the culprit behind pulsar spins is likewise responsible for their magnetic fields. The earlier simulations, the results of which were published in "Pulsar spins from an instability in the accretion shock of supernovae" in the January 2007 edition of Nature, demonstrated that a phenomenon known as the spiral mode occurs when the shock wave expanding from a supernova's core stalls in a phase known as the standing accretion shock instability.
As the expanding shockwave driving the supernova explosion comes to a halt, matter outside the shockwave boundary enters the interior, creating vortices that not only start the star spinning, but also yank and stretch its magnetic fields as well.
This new revelation means two things to astronomers: first, that any rotation that serves as a key driver behind a supernova's magnetism is created via the spiral mode, and second, that not only can the spiral mode drive rotation, but it can also determine the strength of a pulsar's magnetic fields.
Another major finding of the team's simulations is that shear flow from the SASI, or when counter-rotating layers of the star rub against one another during the SASI event, is highly susceptible to turbulence, which can also stretch and strengthen the progenitor's magnetic fields, similar to the expansion of a spring.
These two findings taken together show that core-collapse-supernova magnetic fields can be efficiently generated by a somewhat unexpected source: shear flow-induced turbulence roiling the inner core of the star. "We found that starting with a magnetic field similar to what we think is in a supernova progenitor, this turbulent mechanism is capable of magnifying the magnetic field to pulsar strengths," Endeve said.
The model starts at 4,000 cores, Endeve said, but as the star becomes more chaotic with turbulence and other factors, the simulations are scaled up to 64,000 cores, giving the team a more realistic picture of the magnetic activity in a CCSN. He added that the fact that the time to solution for these hugely varying job sizes is the same due to Jaguar's queue scheduling policy is a "great advantage."
The Daily Galaxy via Oak Ridge National Laboratory
Image above is a remnant of a core-collapsed supernova.