Volume rendering of fluid flows below the supernova shock wave during the operation of the SASI. The fluid velocity streamlines trace out complex flow patterns in the simulation. Credit: Oak Ridge National Laboratory. |
Massive stars are inherently violent creatures—they
burn, they churn, they turn, all the while creating and held hostage by
constantly changing magnetic fields of almost unfathomable strength.
And, eventually, they explode, littering the universe
with the elements of life as we know it: hydrogen, oxygen, carbon, etc.
Everything including ourselves is the result of some star’s violent demise. And
no stars do it better than those that will one day become core-collapse
supernovas (CCSNs), or stars greater than eight solar masses. But the evolution
and nature of these elemental fountains is still a mystery, one of the greatest
unsolved problems in astrophysics. However, perhaps not unsolved for long. 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.
Essentially, said Eirik Endeve, lead author of the
team’s latest paper, researchers want to know how these magnetic fields are
created and how they impact the explosions of these massive stars. A recent
suite of simulations allowed the team to address some of the most fundamental
questions surrounding the magnetic fields of CCSNs. Its findings were published
in The Astrophysical Journal. In
untangling the mystery surrounding these stars’ powerful magnetic fields,
researchers could ultimately explain a great deal as to why these stellar
giants evolve into elemental firecrackers.
In an effort to locate the source of the magnetic
fields, the team simulated a supernova progenitor using tens of millions of hours on Jaguar. The process revealed that we still have much to learn
when it comes to how these stellar marvels operate.
Rotation schmotation
Collapsed supernova remnants are commonly known as pulsars, and when it comes
to magnetic fields, pulsars are the top players 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.
Interestingly, this finding 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 (standing accretion shock instability), 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 CCSN
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 GenASiS of magnetic fields
The team used the General Astrophysical Simulation System to study the
evolution of the progenitor’s magnetic fields. GenASiS, under development by
Christian Cardall, Reuben Budiardia, Endeve and Mezzacappa at ORNL and Pedro
Marronetti at Florida
Atlantic Univ.,
features a novel approach to neutrino transport and gravity and makes fewer
approximations than its earlier counterpart, which assumed CCSNs were perfectly
spherical.
The simulations essentially solved a series of
magnetohydrodynamic equations, or equations that describe the properties of
electrically conductive fluids. After setting the initial conditions, the team
ran several models at low and high resolutions, with the highest-resolution
models taking more than a month to complete. Initially, Endeve said, they were
run at lower resolutions, but very little significant activity occurred.
However, as they ramped up the resolution, things got interesting.
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 facilities here are
excellent,” said Endeve, adding that the center’s high-performance storage
system is very important to the team’s research, as one model produces hundreds
of terabytes of data. “We have also received a lot of help from the
visualization team, especially Ross Toedte, and the group’s liaison to the
OLCF, Bronson Messer.”
The team will next incorporate
sophisticated neutrino transport and relativistic gravity, which will give it
an even more realistic picture of CCSNs. However, to make such a powerful code
economical, said Endeve, it will need to employ an adaptive mesh. And it will
no doubt require Jaguar’s computing power.