October 1, 2014
Anthony Mezzacappa, director of the University of Tennessee–Oak Ridge National Laboratory Joint Institute for Computational Sciences, and a team of computational astrophysicists are conducting one of the largest supernova simulations to date on ORNL’s “Titan” supercomputer. Titan, which is a hybrid Cray® XK7™ supercomputer, is managed by the Oak Ridge Leadership Computing Facility on behalf of the Department of Energy. Dr. Mezzacappa answers our questions about his team’s work on Titan.
Cray Titan at ORNL
Q: Why is understanding what triggers a supernova explosion so important?
A: Supernovae are ultimately responsible for why you and I are here. The class of supernova that our team studies is known as core-collapse supernovae [Type II], and this type of supernova is arguably the most important source of elements in the universe. Core-collapse supernovae are the death throes of massive stars (by massive stars, I’m referring to stars of eight to 10 solar masses and greater). Supernovae are basically stellar explosions that obliterate these stars, leaving the core behind. They are responsible for the lion’s share of elements in the periodic table between oxygen and iron, including the oxygen you breath and the calcium in your bones, and they are believed to be responsible for half the elements heavier than iron. So through supernova explosions, we’re tied to the cosmos in an intimate way.
Twenty years ago, astronomers witnessed one of the brightest stellar explosions in more than 400 years. The titanic supernova, called SN 1987A, blazed with the power of 100 million suns for several months following its discovery on Feb. 23, 1987. Observations of SN 1987A, made over the past 20 years by NASA’s Hubble Space Telescope and many other major ground- and space-based telescopes, have significantly changed astronomers’ views of how massive stars end their lives. Astronomers credit Hubble’s sharp vision with yielding important clues about the massive star’s demise.
This Hubble telescope image shows the supernova’s triple-ring system, including the bright spots along the inner ring of gas surrounding the exploded star. A shock wave of material unleashed by the stellar blast is slamming into regions along the inner ring, heating them up, and causing them to glow. The ring, about a light-year across, was probably shed by the star about 20,000 years before it exploded.
Q: Why is supernova research critical to the progression of astrophysics?
A: In addition to releasing a lot of the elements that make up ourselves and the nature around us, core-collapse supernovae give birth to neutron stars, which can become pulsars or black holes. So these supernovae are also responsible for the birth of other important objects in the universe that we want to understand.
Another reason to study supernovae as a key component of astrophysics is that we can actually use supernovae as nuclear physics laboratories. With supernovae, we’re dealing with very high-density physics, with systems that are rich in neutrons and conditions that are difficult to produce in a terrestrial lab. We can use supernova models in conjunction with observations to understand fundamental nuclear physics.
In all these ways, the “supernova problem,” as we call it, is certainly one of the most important and most challenging problems in astrophysics being answered today.
Q: Back in 2003, what role did the simulations done on “Phoenix,” the Cray® X1E™ supercomputer, have on supernova research?
A: The simulations back in 2002, which we published in 2003, led to the discovery of the SASI, or standing accretion shock instability.
Phoenix was a magnificent machine, and we got a lot of science out of it. On Phoenix, we discovered the SASI and learned that the supernova shock wave, which generates the supernova, is unstable and this instability distorts its shape. The shock wave will become prolate or oblate (cigar-like or pancake-like), which has important ramifications for how these stars explode.
I think if you take a look at supernova theory from about 1980 and onward, the results we see in our 2D and basic 3D models suggest the SASI is the missing link in obtaining supernova explosions in models that have characteristics commensurate with observations.
Q: The work in 2003 unlocked the key SASI simulation result that was recently proven through observation. Can you explain the importance of that breakthrough now?
A: As an x-ray observatory, NuSTAR — which delivered these supporting observations — can see the x-rays emitted from the decay of titanium-44. The reason titanium-44 is so important is because it is produced very deep in the explosion, so it can provide more information about the explosion mechanism than other radiative signatures.
The map of the titanium-44 x-rays gave researchers a map of the explosion and, as such, it gave us a fingerprint, if you will, of the explosion dynamics, which was consistent with the active presence of the SASI. This is a rare example of a computational discovery being made before there was observational evidence to support it because these latest NuSTAR observations occurred a decade after the SASI was simulated on Phoenix. I think computational discovery is likely to happen more often as models develop and the machines they run on develop with them.
Q: There are still some nuances in supernova research that aren’t explained by SASI. What is being done to fill in those gaps?
A: Since the SASI was discovered, all supernova groups have considered the SASI an integral part of supernova dynamics. There is some debate on its importance to the supernova explosion mechanism. Some experts believe it’s there but it’s subcritical to other phenomenon; however, everyone believes it’s there and needs to be understood. I think, when all is said and done, we’ll find the SASI is integral to the explosion mechanism.
The key thing is that Mother Nature works in three dimensions, and earlier simulations have been in 2D. The simulation we are running on Titan now is among the first multiphysics, 3D simulations ever performed.
It’s not even a nuance so much as if you’re trying to understand the role of the SASI and other parts of the explosion mechanism, it has to be done in 3D — an endeavor which is only now beginning.
Q: How is working with Titan unlocking better ways to simulate supernova activity?
A: Unlike earlier 2D simulations, the Titan simulations will model all the important physical properties of a dying massive star in 3D, included gravity, neutrino transport and interaction, and fluid instability.
For gravity, we’re modeling gravitational fields dictated by the general relativistic theory of gravity (or [Albert] Einstein’s theory of gravity). It’s very important that models include calculations for relativistic gravity rather than Newtonian gravity, which you would use to understand the orbits of the planets around the sun, for instance, although even here a deeper description in terms of Einstein’s theory of gravity can be given.
Second, the model includes neutrinos. We believe neutrinos actually power these explosions. They are nearly massless particles that behave like radiation in this system and emerge from the center of the supernova. The center of the supernova is like a neutrino bulb radiating at 1045 watts, and it’s energizing the shock wave by heating the material underneath the wave. There’s a lot of energy in neutrinos, but you only have to tap into a fraction of that energy to generate a supernova.
So neutrinos likely power these explosive events, and their production, transport and interaction with the stellar material has to be modeled very carefully.
Finally, the stellar material is fluid-like, and because you have a heat source (the neutrinos) below that stellar material, convection is going to occur. If you heat a pot of water on the stove, the bubbles that occur during boiling are less dense than the water around them — that’s an instability and that’s why those bubbles rise. Convection is a similar instability that develops. The shock wave is a discontinuity in the stellar fluid, and the SASI is an instability of this shock wave. So convection and the SASI are both operative and are the main fluid instabilities we must model.
Those are the main components. There are other properties — rotation, magnetic fields, thermonuclear reactions and more — that are important for understanding the formation of elements as well as the explosion mechanism, and these will all be modeled on Titan.
Q: What are some of the core goals that make up the INCITE project?
A: The current Titan simulation is representative of supernovae that originate in stars of about 15 solar masses. Later, we will do other runs on Titan at different solar masses — 10, 20, 25. Fifteen solar masses is in the middle of the range of stellar masses we believe result in supernovae, and we’ll compare it to observed supernovae whose progenitor mass is determined to have been at or near 15 solar masses.
This INCITE project is focusing on how the explosion mechanism works, which is not just limited to the SASI. When we run the model we’ll wait to see: Does it explode? If it does, was it a weak or a robust explosion? Was the explosion energy commensurate with observed energies for stars of that solar mass? What kind of neutron star is left behind after the explosion?
Q: If you had to sum up the value of this supernova research, considering everything that has been learned from 2003′s simulations to today, what would you say has been the most important lesson?
A: I would say the most important lesson is that computation is a critical mode of discovery. It is arguably the best tool to understand phenomena that are nonlinear and have many interconnected components. It is very difficult to understand phenomena like core-collapse supernovae analytically with pencil and paper. The SASI had to be discovered computationally because it’s a nonlinear phenomenon. Computation is not just about getting the details. You don’t go into computation knowing the answer but wanting to get the details; there is discovery and surprise in computation. And there’s no better example of that than the discovery of the SASI.
In addition to his role as the director of ORNL’s Joint Institute for Computational Sciences, Anthony Mezzacappa is the Newton W. and Wilma C. Thomas chair and a professor in the department of physics and astronomy at the University of Tennessee. He is also ORNL corporate fellow emeritus. The team simulating a core-collapse supernova on Titan includes Mezzacappa, Steve Bruenn of Florida Atlantic University, Bronson Messer and Raph Hix of ORNL, Eric Lentz and Austin Harris of the University of Tennessee, Knoxville, and John Blondin of North Carolina State University.
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