Binary White Dwarf Mergers

The origins of thermonuclear supernovae remain poorly understood--a troubling fact, given their importance in astrophysics and cosmology. A leading theory posits that these events arise from the merger of white dwarfs in a close binary system. In this study we examine the possibility of prompt ignition, in which a runaway fusion reaction is initiated in the early stages of the merger. We present a set of three-dimensional white dwarf merger simulations performed with the help of a high-resolution adaptive mesh refinement hydrocode. We consider three binary systems of different mass ratios composed of carbon/oxygen white dwarfs with total mass exceeding the Chandrasekhar mass. We additionally explore the effects of mesh resolution on important simulation parameters. We find that two distinct behaviors emerge depending on the progenitor mass ratio. For systems of components with differing masses, a boundary layer forms around the accretor. For systems of nearly equal mass, the merger product displays deep entraintment of each star into the other. We closely monitor thermonuclear burning that begins when sufficiently dense material is shocked during early stages of the merger process. Analysis of ignition times lead us to conclude that for binary systems with components of unequal mass whose combined mass is close to the Chandrasekhar limit, there is a negligible chance of prompt ignition. Simulations of similar systems with a combined mass of 2 solar masses suggest that prompt ignition may be possible, but require further study using higher-resolution. The system with components of nearly equal mass does not seem likely to undergo prompt ignition, and higher resolution simulations are unlikely to change this conclusion. We additionally find that white dwarf merger simulations require high resolution. Insufficient resolution can qualitatively change simulation outcomes, either by smoothing important fluctuations in density and temperature, or by altering the dynamics of the system such that additional physics processes, such as gravity, are incorrectly represented.