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NERSC Initiative for Scientific Exploration (NISE) 2010 Awards

Thermonuclear Explosions from White Dwarf Mergers

Tomasz Plewa, Florida State University

Associated NERSC Project: Supernova Explosions in Three Dimensions (m461), Principal Investigator: Tpmasz Plewa

NISE Award: 500,000 Hours
Award Date: April 2010

We propose to study a formation and evolution of Type Ia supernovae originating from binary white dwarfs. This is an alternative evolutionary channel producing (super) Chandrasekhar mass white dwarfs believed to be progenitors of SN Ias. The importance of this alternative double-degenerate channel has been recently highlighted by Gilfanov and Bogdan (2010, Nature, 463, 924), who found that, contrary to a commonly accepted as standard single degenerate scenario, double-degenerates may account for as much as 95% of all SN Ia events. This is rather unexpected and potentially critically important finding. The proposed study is aimed at increasing our understanding of double-degenerates and thus improving accuracy of supernova-based cosmological predictions. This is of particular importance in the context of the upcoming NASA-DOE Joint Dark Energy Mission (NASA's Beyond Einstein Program, National Research Council, 2007) which will measure thousands of high-redshift supernovae helping to understand physics and origins of the Universe.

We will consider two binary system configurations: first with with 0.6 and 0.9 solar mass components, and the second comprised of two identical 0.9 solar mass white dwarfs. We modeled the evolution of the the unequal mass system in 2008 thanks to the NERSC Scaling Reimbursement program. We had to stop our investigations short of obtaining complete set of results due to insufficient computational resources. Since then we have improved scalability of our code, especially of the self-gravity multigrid solver, and are eager to bring our preliminary study to the conclusion.

The second binary configuration we wish to investigate was recently studied by Pakmor et al. (2010, Nature, 463, 61), who claimed the merger produced a prompt explosion. This result, however, as all white dwarf merger models computed to date, was obtained with help of the Smoothed Particle Hydrodynamics method (SPH). SPH is known to be very diffusive and usually considered as providing a low-cost "first look" at the evolution of complex physical systems. This is especially true in case of hydrodynamics involving steep gradients (i.e. shocks and contact discontinuities). Such gradients are indeed characteristic of stellar mergers with density showing extreme variation at the stellar surface (and especially in case of essentially envelope-free white dwarfs). The hydrodynamic flow also features an accretion shock responsible for compressing and heating stellar material (through converting kinetic energy of the accretion stream into internal energy). SPH stabilizes shocks with help of artificial viscosity resulting in severe loss of numerical resolution at shocks. This is much less of a problem for our high-order Godunov hydro solver.

Thanks to our participation in the NERSC Scaling Reimbursement program, we were able to design and develop a simulation setup and conduct preliminary study of binary white dwarf merger in 0.6+0.9 solar mass configuration. Our preliminary series of 128/64/32 km resolution models established the presence of several interesting flow features. We observe the formation of a hot and turbulent boundary layer at the surface of the more massive component. In this region temperatures approach 1x10^9 K and densities exceed 1x10^5 g/cc. We were not able to confirm higher temperatures reported in models obtained with SPH, although the resolution in our models is perhaps 2 orders of magnitude (in mass) better than in SPH models. Again, such discrepancies between numerical solutions deserve close scrutiny. Also, there is no indication of the Kelvin-Helmholtz instability developing in the boundary layer in SPH models. This stays in clear contrast with our preliminary results and further motivates a careful numerical investigation of the dynamics of the boundary layer.