A visualization of the strong, ordered magnetic field built up by dynamo action in the core of a rapidly rotating, collapsed star. Credit: Moesta et al./Nature
A supercomputer simulation of just 10ms in the collapse of a massive star into a neutron star proves that these catastrophic events, often called hypernovae, can generate the enormous magnetic fields needed to explode the star and fire off bursts of gamma rays visible halfway across the universe.
The simulation demonstrates that as a rotating star collapses, the star and its attached magnetic field spin faster and faster, forming a dynamo that revs the magnetic field to a million billion times the magnetic field of Earth. A field this strong is sufficient to focus and accelerate gas along the rotation axis of the star, creating 2 jets that ultimately can produce oppositely directed blasts of highly energetic gamma rays.
Stellar dynamos generate electrical currents as magnetic fields move through space, while the currents in turn boost the magnetic field, resulting in a feedback loop that produces monster magnetic fields. “A dynamo is a way of taking the small-scale magnetic structures inside a massive star and converting them into larger and larger magnetic structures needed to produce hypernovae and long gamma-ray bursts,” said Philipp Mösta. “That kicks off the process.”
Key to this success was a computer simulation at finer detail than ever before, though one that required 130,000 computer cores operating in parallel over 2 weeks on Blue Waters, one of the most powerful supercomputers in the world.
Astrophysicists like M̦sta are trying to improve their models of what stars do when they reach the ends of their lives, hoping to explain strange cosmic phenomena Рlike gamma-ray bursts and hypernovae that flash 10X brighter than the average supernova Рand understand how some of the very heavy elements found in nature are made.
Observations over the last 50 years have led astronomers to propose bursts are produced during the extremely rare explosions of massive stars – stars 25X the mass of the sun or larger – but details of how such a hypernova generates focused beams of gamma rays are still being worked out. These stellar explosions are classified Type Ic broadline supernovae.
It is thought jets held together by ultra-strong magnetic fields are required to power these explosions but one of the missing links was how a star with a normal magnetic field, like that of the sun, could amplify it a quadrillion (1015) times. One possibility is that energy stored in the rotation of the collapsed star could be transformed into magnetic energy. These strong magnetic fields may also be critical to help accelerate charged particles to a speed and energy able to generate a gamma ray.
MOA: A core-collapse supernova occurs when H fusion in the core stops after all the hydrogen is used up and the star begins to fuse He and then C and O. When the star finally fuses all these elements into iron, fusion stops entirely and the pressure at the core of the star can no longer support the gravitational weight of the surrounding material. Within 1s, the inner star out to a radius of about 1,500 km collapses to a neutron star about 10 to 15 kilometers across, containing the mass of about 1.4 suns >> creates outward-moving shock wave that plows into the outer layers of the star. As the inner star collapses to a neutron star, it increases its spin.
Theorists have attempted to explain how massive, rotating stars generate strong magnetic fields after they have collapsed by a process called magnetorotational instability: Layers of the star rotate at different speeds, creating turbulence that molds the embedded magnetic fields into kilometer-wide magnetic flux tubes much like magnetic flares on the sun. But can this process generate the much larger-scale magnetic fields needed to drive an explosion?
“What we have done are the first global extremely high-resolution simulations of this that actually show that you create this large global field from a purely turbulent one,” Mösta said. “The simulations also demonstrate a mechanism to form magnetars, neutron stars with an extremely strong magnetic field, which may be driving a particular class of very bright supernovae.”
Quataert compares the process to how small-scale turbulence in Earth’s atmosphere coalesces into large-scale hurricanes.
Mösta and his colleagues found that the key to this process in a rapidly rotating neutron star is a shear zone about 15 to 35 kilometers from the star where the different layers are rotating at very different speeds, causing turbulence large enough to create a dynamo.
Mösta is working on simulations that encompass more than 10 milliseconds of the star’s evolution after collapse, or “post-bounce,” to better understand how the collapsing matter and outflowing material interact with the swirling magnetic fields.
http://www.caltech.edu/news/simulation-shows-key-building-powerful-magnetic-fields-48830 http://phys.org/news/2015-11-link-turbulence-collapsing-star-hypernova.htmljCp
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