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Browsing by Author "Lamb, D. Q."

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    Capturing the Fire: Flame Energetics and Neutronization For Type Ia Supernova Simulations
    (2007-02-10) Calder, A. C.; Townsley, D. M.; Seitenzahl, I. R.; Peng, F.; Messer, O. E. B.; Vladimirova, N.; Brown, E. F.; Truran, J. W.; Lamb, D. Q.; University of Alabama Tuscaloosa
    We develop and calibrate a realistic model flame for hydrodynamic simulations of deflagrations in white dwarf (Type Ia) supernovae. Our flame model builds on the advection-diffusion-reaction model of Khokhlov and includes electron screening and Coulomb corrections to the equation of state in a self-consistent way. We calibrate this model flame—its energetics and timescales for energy release and neutronization—with self-heating reaction network calculations that include both these Coulomb effects and up-to-date weak interactions. The burned material evolves postflame due to both weak interactions and hydrodynamic changes in density and temperature.We develop a scheme to follow the evolution, including neutronization, of the NSE state subsequent to the passage of the flame front. As a result, our model flame is suitable for deflagration simulations over a wide range of initial central densities and can track the temperature and electron fraction of the burned material through the explosion and into the expansion of the ejecta.
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    The Detonation Mechanism of the Pulsationally Assisted Gravitationally Confined Detonation Model of Type Ia Supernovae
    (2012-11-01) Jordan, G. C. IV; Graziani, C.; Fisher, R. T.; Townsley, D. M.; Meakin, C.; Weide, K.; Reid, L. B.; Norris, J.; Hudson, R.; Lamb, D. Q.; University of Alabama Tuscaloosa
    We describe the detonation mechanism composing the “pulsationally assisted” gravitationally confined detonation (GCD) model of Type Ia supernovae. This model is analogous to the previous GCD model reported in Jordan et al.; however, the chosen initial conditions produce a substantively different detonation mechanism, resulting from a larger energy release during the deflagration phase. The resulting final kinetic energy and 56Ni yields conform better to observational values than is the case for the “classical” GCD models. In the present class of models, the ignition of a deflagration phase leads to a rising, burning plume of ash. The ash breaks out of the surface of the white dwarf, flows laterally around the star, and converges on the collision region at the antipodal point from where it broke out. The amount of energy released during the deflagration phase is enough to cause the star to rapidly expand, so that when the ash reaches the antipodal point, the surface density is too low to initiate a detonation. Instead, as the ash flows into the collision region (while mixing with surface fuel), the star reaches its maximally expanded state and then contracts. The stellar contraction acts to increase the density of the star, including the density in the collision region. This both raises the temperature and density of the fuel–ash mixture in the collision region and ultimately leads to thermodynamic conditions that are necessary for the Zel’dovich gradient mechanism to produce a detonation. We demonstrate feasibility of this scenario with three three-dimensional (3D), full star simulations of this model using the FLASH code. We characterized the simulations by the energy released during the deflagration phase, which ranged from 38% to 78% of the white dwarf’s binding energy. We show that the necessary conditions for detonation are achieved in all three of the models.