Critical Conditions For The Detonation of Helium Matter: An Investigation Into The Destruction Of Wh
Physical and Biological Sciences
Physics/Astrophysics
Non-technical description:
Type Ia supernovae (hereafter SNe Ia) are among the most powerful explosions in the known Universe, so it's not surprising that astrophysicists have been interested in them since the first sighting of one (circa 1000 a.d.). Beyond that inherent interest, however, SNe Ia are valuable because their light properties are such that they can be used to determine extreme distances in space. It was precisely this useful property that lead to the discovery of dark energy (for which the 2011 Nobel prize was awarded). The problem is, we don't fully understand how these explosive events come to be. It is a consensus within the astrophysics community that the progenitor systems involve at least one white dwarf star, but beyond that little is known for certain.
Many models for the initiation of SNe Ia rely on the detonation of helium (He) gas via nuclear fusion. To date, the has been no systematic attempt to determine the thermodynamical conditions under which He can detonate. We provide just such a study by performing several thousand hydrodynamical simulations using UCSC's supercomputer cluster Laozi. We do not solve the SN Ia problem, but our results do offer valuable insights into the nature of possible progenitor systems. We find that He detonations can occur in a wider range of low mass white dwarfs than previously thought possible, and that these detonations have the ability to produce a diversity of nucleosynthetic outcomes. Because atomic abundances are among the most valuable observables in astronomy, our work can be directly compared with observations of SNe. To illustrate the robustness of our results, we compare our calculated nuclear yields with observations of SN 2005E, and find that this remnant is consistent with a low mass white dwarf progenitor that underwent He detonations. This conclusion is in agreement with the predictions of the authors of the SN 2005E paper.
Here is the slightly more technical abstract from the thesis:
"The nature of type Ia supernova (SN Ia) progenitors is a mystery that has vexed the astrophysics community for more than a century. Several models that attempt to explain the SN Ia phenomenon rely on the production of a self-sustained detonation powered by the nuclear fusion of helium (He) gas. Until now, no systematic investigation into the production of He detonations has been conducted. We perform several thousand super- computer simulations at a variety of initial density and temperature combinations and find critical length scales for the initiation of He detonations that range between 1 – 10^10 cm. We find that WDs with as little mass as 0.24 M_sun can be detonated if their mass is thermalized, violating the limits of what was previously thought possible. The initiation lengths obtained are smaller than theoretically predicted. Certain systems therefore may not reach a steady state of heavy element production within the time it takes a detonation to traverse the object, resulting in a diversity of nucleosynthetic outcomes. In support of this hypothesis, we demonstrate that incomplete burning will occur in the majority of He WD detonations and that intermediate mass elements dominate for many of these events. We anticipate that a measure of the elemental abundances of helium-rich thermonuclear explosions can potentially be used to constrain the nature of the progenitor systems. We demonstrate the robustness of our results by constraining the progenitor system of SN 2005E."
