From Soundwaves to Supernovae

Once every 30 to 50 years in our galaxy—and then just for a few milliseconds—an exploding star known as a core-collapse (Type II) supernova emits as much energy in neutrinos as is emitted in photons by all the other stars in the Universe combined.

Supernovae have been documented for 1,000 years, and astrophysicists know a lot about how they form, what happens during the explosion and what’s left afterward. But for the past 40 years, one problem has dogged astrophysicists—what is the mechanism that actually triggers the massive explosion? Hydrodynamic, neutrino, convective, viscous, and magnetic mechanisms for driving core-collapse supernova explosions have all been proposed and investigated.

One thing that is known is that Type II supernovae produce neutrinos, particles with very little mass which travel through space and everything in their path. Neutrinos carry energy from the deep interior of the star, which is being shaken around like a jar of supersonic salad dressing, and deposit the energy on the outer region. One theory holds that if the neutrinos deposit enough energy throughout the star, this may trigger the explosion.

To study this, a group led by Adam Burrows, professor of astronomy at the University of Arizona and a member of the SciDAC Computational Astrophysics Consortium, developed codes for simulating the behavior of a supernova core in two dimensions. While a 3D version of the code would be optimum, it would take at least five more years to develop and would require up to 300 times as much computing time. As it was, the group ran 1.5 million hours of calculations at NERSC.

But the two-dimensional model is suitable for Burrows’ work, and the instabilities his group is interested in studying can be seen in 2D. What they found was that there is a big overturning motion in the core, which leads to wobbling, which in turn creates sound waves. These waves then carry energy away from the core, depositing it farther out near the mantle.

According to Burrows, these oscillations could provide the power that puts the star over the edge and causes it to explode. To imagine what such a scenario would look like, think of a pond into which rocks are thrown, causing waves to ripple out. Now think of the pond as a sphere, with the waves moving throughout the sphere. As the waves move from the denser core to the less dense mantle, they speed up. According to the model, they begin to crack like a bullwhip, which creates shockwaves. It is these shockwaves, Burrows believes, which could trigger the explosion (Figures 1 and 2).

Figure 1. A 2D rendition of the entropy field of the early blast in the inner 500 km of an exploding supernova. Velocity vectors depict the direction and magnitude of the local flow. The bunching of the arrows indicates the crests of the sound waves that are escalating into shock waves. These waves are propagating outward, carrying energy from the core to the mantle and helping it to explode. The purple dot is the protoneutron star, and the purple streams crashing in on it are the accretion funnels. (Click figure for larger view)		Figure 2. This shell of isodensity contours, colored according to entropy values, shows simultaneous accretion on the top and explosion on the bottom. The inner green region is the blast, and the outer orange region is the unshocked material that is falling in. The purple dot is the newly formed neutron star, which is accumulating mass through the accretion funnels (in orange). (Click figure for larger view)

So, what led the team to this new model of an acoustic triggering mechanism? They came up with the idea by following the pulsar—the neutron star which is the remains of a supernova. They wanted to explore the origin of the high speed which pulsars seem to be born with, and this led them to create a code that allowed the core to move. However, when they implemented this code, the core not only recoiled, but oscillated and generated sound waves.

The possible explosive effect of the oscillations had not been considered before because previous simulations of the conditions inside the core used smaller time steps, which consumed more computing resources. With this limitation, the simulations ran their course before the onset of oscillations. With SciDAC support, however, Burrows’ team was able to develop new codes with larger time steps, allowing them to model the oscillations for the first time.

In the paper resulting from this research,[1] neutrino transfer is included as a central theme in a 2D multi-group, multi-neutrino, flux-limited transport scheme—the first truly 2D neutrino code with results published in the archival literature. The results are approximate but include all the important components.

Calling the simulation a “real numerical challenge,” Burrows said the resulting approach “liberated the inner core to allow it to execute its natural multidimensional motion.” This motion led to the excitation of the core, causing the oscillations at a distinct frequency.

The results look promising, but as is often the case, more research is needed before a definitive mechanism for triggering a Type II supernova is determined. For example, if a simulation with better numerics or three dimensions produces a neutrino triggering mechanism that explodes the star earlier, then the acoustic mechanism would be aborted. Whether this happens remains to be seen and is the subject of intense research at NERSC and elsewhere.

“The problem isn’t solved,” Burrows said. “In fact, it’s just beginning.”

[1] A. Burrows, E. Livne, L. Dessart, C. D. Ott, and J. Murphy, “A new mechanism for core-collapse supernova explosions,” Astrophysical Journal 640, 878 (2006).