We develop an improved method for tracking the nuclear flame during the deflagration phase of a Type Ia supernova, and apply it to study the variation in outcomes expected from the gravitationally confined detonation (GCD) paradigm. A simplified 3-stage burning model and a non-static ash state are integrated with an artificially thickened advection-diffusion-reaction (ADR) flame front in order to provide an accurate but highly efficient representation of the energy release and electron capture in and after the unresolvable flame. We demonstrate that both our ADR and energy release methods do not generate significant acoustic noise, as has been a problem with previous ADR-based schemes. We proceed to model aspects of the deflagration, particularly the role of buoyancy of the hot ash, and find that our methods are reasonably well-behaved with respect to numerical resolution. We show that if a detonation occurs in material swept up by the material ejected by the first rising bubble but gravitationally confined to the white dwarf (WD) surface (the GCD paradigm), the density structure of the WD at detonation is systematically correlated with the distance of the deflagration ignition point from the center of the star. Coupled to a suitably stochastic ignition process, this correlation may provide a plausible explanation for the variety of nickel masses seen in Type Ia Supernovae.

We report the results of a series of three-dimensional (3D) simulations of the deflagration phase of the gravitationally confined detonation mechanism for Type Ia supernovae. In this mechanism, ignition occurs at one or several off-center points, resulting in a burning bubble of hot ash that rises rapidly, breaks through the surface of the star, and collides at a point opposite the breakout on the stellar surface. We find that detonation conditions are robustly reached in our 3D simulations for a range of initial conditions and resolutions. Detonation conditions are achieved as the result of an inwardly directed jet that is produced by the compression of unburnt surface material when the surface flow collides with itself. A high-velocity outwardly directed jet is also produced. The initial conditions explored in this paper lead to conditions at detonation that can be expected to produce large amounts of 56 Ni and small amounts of intermediate-mass elements. These particular simulations are therefore relevant only to high-luminosity Type Ia supernovae. Recent observations of Type Ia supernovae imply a compositional structure that is qualitatively consistent with that expected from these simulations.

We analyze the sensitivity of the flame propagation in a Chandrasekhar mass white dwarf to initial conditions during the subsonic burning phase (deflagration), using 2D simulations of the full WD. Results are presented for a wide variety of initial flame distributions including central and off-center single point and multi-point, simultaneous and non-simultaneous, ignitions. We also examine the effects of convective velocity field which should exist at the core before the thermo-nuclear runaway.Our main conclusion suggests that the amounts of burning products and their distributions through the deflagration phase are extremely sensitive to initial conditions, much more sensitive than presented in previous studies.In particular, we find that more complex configurations such as even slight off-center ignitions, non-simultaneous multi-point ignitions and velocity fields tend to favor solutions in which individual plumes rise faster than the bulk of a typical Rayleigh-Taylor driven, unstable burning front. The difference to previous calculations for an octant of a WD may be understood as a consequence of the suppression of l=1,2 modes. Our results are consistent with full star calculations by the Chicago group. Moreover, the total amount of nuclear burning during the phase of subsonic burning depends sensitively on the initial conditions and may cause the WD to pulsate or to become unbound. We discuss the implications of the results on current models for Type Ia SNe, limitations imposed by the 2-D nature of our study, and suggest directions for further study.Subject headings: supernovae, hydrodynamics IntroductionThe last decade has witnessed an explosive growth of high-quality data for thermonuclear explosions of a White Dwarf Star (WD), the Type Ia Supernovae (SNe Ia).Advances in computational methods provide new insights into the physics of the phenomenon and a direct, quantitative link between observables and explosion physics.Both trends combined provided spectacular results, allowed to address, to identify specific problems and to narrow down the range of scenarios. However, with the advances came the realization that observational constrains seem to be at odds with the most elaborated calculations for deflagration fronts, one of the central parts in the currently most favored model, the thermonuclear explosion a Chandrasekhar mass WD.The M Ch scenario requires an initial phase of pre-expansion because, otherwise, almost the entire WD would burn to 56 Ni, in contradiction to the observation which show a significant amount of intermediate mass elements, namely O, Si, and S. This pre-expansion is commonly believed to occur during an initial phase of a slow deflagration that preserves the WD structure but decreases the binding energy. In absence of strong mixing, the pre-expansion depends mainly on the total amount of burning but hardly on its actual rate Dominguez & Höflich(2000). Successful spherical models need either a rapidly increasing deflagration speed and no radial mixing (similar to W7 Nomoto et al. 1984...

Two dimensional hydrodynamical simulations of convective oxygen burning shell in the presupernova evolution of a 20 solar-mass star are extended to later times. We used the VULCAN code to simulate longer evolution times than previously possible. Our results confirm the previous work of Bazan and Arnett (98) over their time span of 400s. However, at 1200s, we could identify a new steady state that is significantly different than the original one dimensional model. There is considerable overshooting at both the top and bottom boundaries of convection zone. Beyond the boundaries, the convective velocity falls off exponentially, with excitation of internal modes. The resulting mixing greatly affect the evolution of the simulations. Connections with other works of simulation of convection, in which such behavior is found in a different context, are discussed.Comment: 8 pages including 15 figures, accepted for publication in Ap

Understanding the nature of turbulent flows remains one of the outstanding questions in classical physics. Significant progress has been recently made using computer simulation as an aid to our understanding of the rich physics of turbulence. Here, we present both the computer science and the scientific features of a unique terascale simulation of a weakly compressible turbulent flow that includes tracer particles. (Terascale refers to performance and dataset storage use in excess of a teraflop and terabyte, respectively.) The simulation was performed on the Lawrence Livermore National Laboratory IBM Blue Gene/Le system, using version 3 of the FLASH application framework. FLASH3 is a modular, publicly available code designed primarily for astrophysical simulations, which scales well to massively parallel environments. We discuss issues related to the analysis and visualization of such a massive simulation and present initial scientific results. We also discuss challenges related to making the database available for public release. We suggest that widespread adoption of an open dataset model of high-performance computing is likely to result in significant advantages for the scientific computing community, in much the same way that the widespread adoption of open-source software has produced similar gains over the last 10 years.

Two-dimensional hydrodynamical simulations were made to calibrate the mixing length parameter for modeling red giant's convective envelope. As was briefly reported in Asida & Tuchman 1997, a comparison of simulations starting with models integrated with different values of the mixing length parameter, has been made. In this paper more results are presented, including tests of the spatial resolution and Large Eddy Simulation terms used by the numerical code. The consistent value of the mixing length parameter was found to be 1.4, for a red giant of mass 1.2M ⊙ , core mass of 0.96M ⊙ , luminosity of 200L ⊙ , and metallicity Z = 0.001.Subject headings: convection -methods: numerical -stars: AGB and post-AGB -stars: atmosphere Alternatives to the usual mixing length used in MLT (which is a free parameter times

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