Studying Core-collapse Supernovae to Determine the Mechanism That Spurs Their Explosion
By Jacob Pieper
Aside from marking the death of the most massive stars in the universe, core-collapse supernovae also exhibit the most-energetic explosions. In turn, these supernova explosions disperse many of the elements known to humankind. These same elements, which have been strewn violently into our universe, make life as we know it possible. Understandably then, the study of these supernovae is an important factor not only in astrophysical research but also our own origin stemming from the Big Bang itself.
While most of the phenomena of core-collapse supernovae are well understood, the source of energy for the explosion itself remains a mystery. The explosive mechanism within these supernovae is the current research focus for principal investigator Adam Burrows and colleagues Joshua Dolence and Jeremiah Murphy at Princeton University.
The death of these enormous stars, which generally have a mass greater than eight times that of our sun, begins when thermonuclear fusion can no longer maintain the balance between the heated gases pushing outward on the stellar surface and gravity pulling in. Consequently, in less than 1 second (and after 10 million years of quiescent existence), the star's core collapses and then rebounds violently outward, creating a shockwave. This is the point in the process—just before the supernova explodes—where Burrows and his team are focusing their study.
Burrows explains that every good calculation being done on this topic indicates that after the shock wave bounces outward, the event stalls. "The last 20 to 30 years of theoretical effort have been devoted to understanding how this shock wave can be revived into [an] explosion," he says.
Recently, Burrows and his colleagues have made great strides in determining what this explosive mechanism may be. Through their use of the Kraken supercomputer and other resources from the National Institute for Computational Sciences (NICS), they have been able to successfully demonstrate the qualitative differences between simulating supernova explosions in 2D and 3D.
Before Burrows' research, an axial sloshing motion observed in 2D simulations was assumed to be important in determining the explosive mechanism. The sloshing motion inside the exploding star can be compared to the sloshing and bubbling seen in a pot of boiling water. But by using Kraken for 3D simulations in conjunction with the CASTRO (Compressible ASTROphysics) code developed by Burrows and his team, the researchers have shown that in reality the sloshing is of less importance to the explosion than neutrino-driven convection.
Burrows and his colleagues are investigating the theory that subatomic, electrically neutral neutrinos might be capable of re-energizing the explosion. In other words, the neutrinos may be able to sufficiently heat the material within the stalled shock wave and jump-start the supernova explosion.
One function of the CASTRO code is to allow the modeling of the multidimensional hydrodynamics and the radiative transport of the star in a 3D simulation. Those phenomena, which occur behind the shock wave—and influence the heating of the neutrinos—may help restart the explosion after the stall, Burrows says.
Hydrodynamics in this field simply refers to the movement of the superheated gases within the stellar surface. Radiative transport refers to the transfer of energy within the star as electromagnetic radiation. Yet, when these dynamic phenomena are simulated in multiple dimensions, as is the case in core-collapse supernovae, they each become infinitely more complex. Thus, as Burrows explains, "The problem is of a quintessentially supercomputer character, because the flow is turbulent, and radiative transfer is difficult computationally."
Thus, by using Kraken, the researchers are able to simulate the multidimensional hydrodynamics and the radiative transport that influence the heating of neutrinos in the star's core that current theory suggests may be responsible for re-energizing supernova explosions—spectacular cosmic blasts that briefly outshine the surrounding galaxy and disburse essential life-sustaining elements. Without the types of HPC capabilities offered by NICS and the CASTRO code, Burrows notes, the 3D simulations providing the insight about neutrinos might not be possible.
- "An Investigation into the Character of Pre-Explosion Core-Collapse Supernova Shock Motion" (Adam Burrows, Josh Dolence, & Jeremiah Murphy), 2012, Ap.J., 759, 5 (arXiv:1204.3088).
- "The Dominance of Neutrino-Driven Convection in Core-Collapse Supernovae" (J.W. Murphy, J.C. Dolence, & A. Burrows), Ap.J., 771, 52, 2013.
- "Dimensional Dependence of the Hydrodynamics of Core-Collapse Supernovae" (J. Dolence, A. Burrows, J. Murphy, & J. Nordhaus), Ap.J., 765, 110, 2013 (arXiv:1210.5241).
- "Perspectives on Core-Collapse Supernova Theory" (Adam Burrows), Rev. Mod. Phys., 85, 245, 2013 (arXiv:12104921) (cover article).
Article posting date: 4 March 2014
About NICS: The National Institute for Computational Sciences (NICS) operates the University of Tennessee supercomputing center, funded in part by the National Science Foundation. NICS is a major partner in NSF’s Extreme Science and Engineering Discovery Environment, known as XSEDE.