UC Berkeley

In double white dwarf (WD) systems with sufficiently short initial orbital periods, angular momentum losses from gravitational wave radiation shrink the orbit and can lead to the merger of the WDs. Simulations of the merger show that the less massive WD is tidally disrupted, forming a disk around the more massive WD. Beginning with output from WD merger simulations, I study the subsequent viscous evolution using multi-dimensional hydrodynamical simulations. I find that the remnants evolve towards a spherical end-state, where the rotationally-supported disk has been converted into a hot, thermally-supported envelope. I then map these results into a stellar evolution code and evolve them over thermal and nuclear timescales. This is a necessary procedure to self-consistently study the long-term outcomes of WD mergers. I apply this to the merger of two carbon-oxygen WDs with a total mass in excess of the Chandrasekhar mass. My work follows the evolution of the remnants for longer than previous calculations and finds alternating episodes of fusion and contraction can lead to the formation and subsequent collapse of an iron core. I also characterize the observable properties of the remnant during this evolution. Additionally, I develop a framework to compute weak reaction rates that are essential in the evolution of massive, accreting WDs. I apply these results to understand the evolution of oxygen-neon WDs towards accretion-induced collapse to a neutron star.