Detonability of Turbulent White Dwarf Plasma: Hydrodynamical Models at Low Densities
The origins of Type Ia supernovae (SNe Ia) remain an unsolved problem of contemporary astrophysics. Decades of research indicate that these supernovae arise from thermonuclear runaway in the degenerate material of white dwarf stars; however, the mechanism of these explosions...
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| Language: | English English |
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Florida State University
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| Online Access: | http://purl.flvc.org/fsu/fd/FSU_FA2016_Fenn_fsu_0071E_13617 |
| Summary: | The origins of Type Ia supernovae (SNe Ia) remain an unsolved problem of contemporary astrophysics. Decades of research
indicate that these supernovae arise from thermonuclear runaway in the degenerate material of white dwarf stars; however, the mechanism of
these explosions is unknown. Also, it is unclear what are the progenitors of these objects. These missing elements are vital components of
the initial conditions of supernova explosions, and are essential to understanding these events. A requirement of any successful SN Ia
model is that a sufficient portion of the white dwarf plasma must be brought under conditions conducive to explosive burning. Our aim is
to identify the conditions required to trigger detonations in turbulent, carbon-rich degenerate plasma at low densities. We study this
problem by modeling the hydrodynamic evolution of a turbulent region filled with a carbon/oxygen mixture at a density, temperature, and
Mach number characteristic of conditions found in the 0.8+1.2 solar mass (CO0812) model discussed by Fenn et al. (2016). We probe the
ignition conditions for different degrees of compressibility in turbulent driving. We assess the probability of successful detonations
based on characteristics of the identified ignition kernels, using Eulerian and Lagrangian statistics of turbulent flow. We found that
material with very short ignition times is abundant in the case that turbulence is driven compressively. This material forms contiguous
structures that persist over many ignition time scales, and that we identify as prospective detonation kernels. Detailed analysis of the
kernels revealed that their central regions are densely filled with material characterized by short ignition times and contain the minimum
mass required for self-sustained detonations to form. It is conceivable that ignition kernels will be formed for lower compressibility in
the turbulent driving. However, we found no detonation kernels in models driven 87.5 percent compressively. We indirectly confirmed the
existence of the lower limit of the degree of compressibility of the turbulent drive for the formation of detonation kernels by analyzing
simulation results of the He0609 model of Fenn et al. (2016), which produces a detonation in a helium-rich boundary layer. We found that
the amount of energy in the compressible component of the kinetic energy in this model corresponds to about 96 percent compressibility in
the turbulent drive. The fact that no detonation was found in the original CO0812 model for nominally the same problem conditions suggests
that models with carbon-rich boundary layers may require higher resolution in order to adequately represent the mass distributions in
terms of ignition times. === A Dissertation submitted to the Department of Scientific Computing in partial fulfillment of the
requirements for the degree of Doctor of Philosophy. === Fall Semester 2016. === November 22, 2016. === Includes bibliographical references. === Tomasz Plewa, Professor Directing Dissertation; Mark Sussman, University Representative; Gordon
Erlebacher, Committee Member; Jorge Piekarewicz, Committee Member; Sachin Shanbhag, Committee Member. |
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