Type Ia (thermonuclear-powered) supernovae are among the brightest explosions in the universe. These events are thought to be white dwarf stars in binary systems that explode as a result of a thermonuclear runaway. Observations using them as "cosmic yardsticks" revealed that the expansion rate of the universe is accelerating and led to the discovery of dark energy.
The goal of the Flash Center's Type Ia supernova (SN Ia) project is to understand these explosions better, and by doing so, help observers use them to determine the properties of dark energy.
The Type Ia supernova research team is using the FLASH computer code and INCITE allocations of computer time on the IBM Blue Gene/P at the ALCF to conduct the first comprehensive, systematic validation of all current SN Ia models.
As an important first step, the team has conducted extensive simulations of buoyancy-driven turbulent nuclear combustion, a physical process that is key to SNe Ia but not fully understood. The goal of the simulations is to provide definite answers to three questions:
The FLASH simulations of turbulent nuclear combustion show that the flame surface is complex at large scales and smooth at small scales, and suggest that the so-called flame polishing lengthÓ is the only length scale on which the burning rate depends. They also show that the transition to distributed burning occurs at a much smaller flame speed in the case of buoyancy-driven turbulent nuclear combustion than in the case of nuclear combustion in the presence of fully developed turbulence. Finally, they show that the nuclear burning rate is smaller than it would be if the turbulence were fully developed. The SN Ia team is using this information to develop a better flame model for use in its large-scale, 3D, whole-star simulations of Type Ia supernovae (SNe Ia).
The team'ss simulations of SNe Ia have produced new insights into how these stars explode, including the discover of a new mechanism, termed the Ògravitationally confined detonation's (GCD) model. They also have led to the discovery of robust signatures for the different SN Ia models, holding the promise that observations can discriminate among the models.
Now the Flash Center team is confronting the light curves and spectra predicted by the simulations with high-quality data from the SDSS-II Supernova Survey team and its collaborators. Preliminary results based on simulations of the explosion phase using FLASH and calculations of the radiation-transfer phase using the Phoenix and Sedona codes show that the light curves predicted by the GCD model can be fit reasonably well by the data-driven models the supernova community uses to fit the light curves of individual SNe Ia, demonstrating that the predicted light curve are similar to those observed. These results also show the light curves predicted by the GCD model are consistent with the relation between peak luminosity and rate of fading that is observed. The results also suggest that the anomalous scatter in the calibration of SNe Ia as cosmological yardsticks may be due in part to the difference in how the intrinsically asymmetric supernovae look when viewed from different directions.
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