Calibration of Type Ia supernova models based on high resolution simulations
|Scientific Discipline||Astrophysics, Supernova Research|
|Principal Investigator(s)||Prof. Dr. Wolfgang Hillebrandt|
|Leading Institution||Max Planck Institute for Astrophysics, Germany|
|DEISA Home Site||RZG|
Project summary and results
Only about once in a century, a "new star" might appear on the night sky, visible to the naked eye, which fades away over some weeks. A sub-group of these events - classified as Type Ia supernovae (SNe Ia) -are among the most energetic explosions in the universe. Being rare in single galaxies such as the Milky Way, the majority of SNe Ia occurs far away. Due to their enormous intrinsic brightness, outshining billions of stars, large telescopes facilitate the observation of supernovae that went off when the universe was only about half its present age.
Therefore, they can be used as "lighthouses" and "standard candles" to determine the geometry and the expansion rate of the universe. Here, one exploits the fact that SNe Ia are (at least by astronomical standards) remarkably uniform. Knowing the intrinsic luminosity, their apparent brightness tells us their distance. These measurements indicated that the universe undergoes an accelerated expansion at present - a result which fits into the theoretical framework of General Relativity, given that a yet unknown form of "dark" energy is assumed to dominate the universe today. Thus, SNe Ia may have given birth to a major revision of our understanding of physics and of the universe.
However, a close look to the sample of wellobserved nearby SNe Ia reveals an intrinsic scatter in their brightnesses which is evidently connected to other properties. Such correlations are used to calibrate the cosmological distance determinations. But, as yet, these are established only empirically. A sound theoretical understanding of the explosion mechanism is therefore one of the great challenges of astrophysics.
Only models that tackle the underlying physics in a self-consistent way are predictive enough to provide answers concerning the reliability of SNe Ia as cosmic distance indicators. A promising approach is pursued at the Max Planck Institute for Astrophysics. By constructing numerical models of SN Ia explosions and by comparing the results with detailed observations of nearby Type Ia supernovae, new insights into the explosion mechanism are gained.
Although there exists consensus about the astrophysical scenario, the exact explosion mechanism is unclear. SNe Ia are attributed to the thermonuclear explosion of White Dwarf stars, of about 1.4 solar masses, which consist of carbon and oxygen. The behavior of such a final event of the star life implies a very complex variety of phenomena that make SN Ia explosions a problem of turbulent combustion much like the burning of fuel in car engines.
The challenge of implementing this scenario into numerical simulations lies in the vast range of relevant length scales. While the radius of the exploding White Dwarf star amounts to ~2000 km, the width of a flame typically is less than a millimeter. In order to model turbulence effects consistently, three-dimensional simulations are inevitable, and it is impossible to resolve the full range of scales with a brute approach. Thus different correlated methods and models have been used to code the solution and the resultant simulation was carried out within the DEISA framework on HPCx, the IBM cluster in the UK using 512 processors. The evolution of the explosion process was obtained using 6403 cells.
The results are illustrated in the figure. Based on this simulation, observables can be predicted and these will then be compared with actual observations to assess the validity of the SN Ia model. This pipeline of post-processing steps is a pan-European effort including research groups from Germany, Italy, Sweden, and Russia. The distributed DEISA infrastructure will continue to provide the resources for this task. Indeed, DEISA is proud to host this project, particularly since its relevance in Cosmology is documented by the choice of the Scientific American newsletter to use its results as the cover story ("How to Blow Up a Star") for the October 2006 issue.
Snapshot from a Type Ia supernova explosion simulation on DEISA. The largest nearspherical shape is the volume rendering of the logarithm of the density which indicates the White Dwarf star, and the inner isosurface represents the thermonuclear flame. Its complex structure results from instabilities and turbulent nature of the flow as the flame progresses from its ignition, near the star's center, towards the surface.