White dwarfs (WDs) are the incredibly dense, burnt-out husks of Sun-like stars after they extinguish their nuclear fuel, and are generally composed of carbon and oxygen (these are called CO white dwarfs, or CO WDs). Having no nuclear reactions of their own, WDs remain inert forever.

It has been well-established, however, that a fraction of CO WDs end their lives in type Ia supernovae, the greatest thermonuclear explosions in the universe. These supernovae are so bright they can be seen from billions of light years away, making them invaluable distance-markers in many fields of astronomy. A conundrum remains, though: why do normally inert CO WDs completely destroy themselves in thermonuclear explosions? The general consensus among researchers is that the WD is triggered by interacting with a companion star, which makes the interior of the WD either too dense or too hot, both of which can trigger runaway nuclear fusion. The exact nature of that trigger remains highly disputed to this day and a diversity of scenarios have been proposed and require further investigation.

An inspiralling white dwarf binary. Image courtesy of NASA.

My thesis work looks into the merger of two CO WDs - a violent event featuring two WDs that tear each other apart by their mutual (tidal) gravitational pull. The specific scenario I'm testing was proposed by my supervisors in 2010, and hinges on the merged object, or "merger remnant", evolving to become hot enough to trigger nuclear fusion. In Zhu et al. 2013, my collaborators and I performed merger simulations for a wide variety of CO WD binaries using the hydrodyamics code Gasoline SPH, and found the properties of the merger remnant have simple dependencies on the masses of the merging CO WDs. Merging CO WDs whose masses differ by less than a tenth of the mass of the Sun lead to hot, rapidly rotating merger remnants. Following the merger, the remnant will rapidly spin down, leading to more heating and compression - potentially enough to start runaway carbon fusion. This makes these "similar-mass" mergers are candidate supernova progenitors.

Temperature time evolution (from left to right, top to bottom) of a merging 0.625 - 0.65 solar mass CO WD binary, simulated in Arepo.

Following that work, I ran merger simulations in the magnetohydrodynamics code Arepo in conjunction with Dr. Rüdiger Pakmor. The results, in Zhu et al. 2015 show exponential magnetic field growth during the final stages of the merger, leading to a remnant with magnetic fields 100,000 more powerful than the strongest fields ever generated in laboratories on Earth. A movie of the merging process and magnetic field amplification can be found here

Series of temperature profiles representing a WD undergoing a nuclear runaway. The colour gradient represents a transition from low (blue) central temperature early-on in the runaway to high (orange) ones later on.

Recently, I've been working on the further evolution of the remnant once nuclear fusion is underway. In stars like our Sun, an increase in gas temperature also increases the pressure, allowing a star to regulate fusion reactions. In WDs, however, the pressure (due to electron degeneracy) is largely independent of temperature, and nuclear fusion only serves to make the WD hotter, which in turn stimulates further fusion. This "nuclear runaway" leads to so much heating that either degeneracy is "lifted" and the WD inflates into a carbon-burning star, or WD explodes. To analyze this phase of the WD's evolution, I created a code to generate sequences of WD models with increasing central temperature, in order to characterize the response WD structure's response. This project will tell us the critical mass above which a runaway leads to an explosion, as well as how this critical mass is altered by the WD's rotation and magnetic field.



Feed courtesy of You can also find my publications on the arXiv site.