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Illuminating Mergers of Carbon-Oxygen White Dwarfs and Their Possible Link to Thermonuclear Supernovae

Chenchong Zhu

Doctor of Philosophy 2016
Graduate Department of Astronomy and Astrophysics, University of Toronto

The merger of two carbon-oxygen white dwarfs (CO WDs) can either create a more massive WD, lead to collapse into a neutron star, or explode spectacularly as a thermonuclear, or Type Ia, supernova (SN Ia). It has traditionally been believed that SNe Ia result only from mergers at or above the Chandrasekhar mass (Mch), as these can subsequently become dense enough to trigger runaway fusion. Recently, however, it has been proposed that the merged product, or "remnant", might instead subsequently ignite fusion from high temperatures. This opens the possibility of SNe Ia arising even from sub-Mch mergers.

To investigate this, I conducted a series of hydrodynamic simulations of the merging process. I first performed simulations spanning the range of possible mergers using the smoothed-particle hydrodynamics code GASOLINE, finding that remnant configurations are roughly homologous for mergers of WDs with the same difference in mass deltaM. In particular, "similar-mass" mergers with deltaM < 0.1 Msolar generate remnants that are heated throughout their dense cores, making them candidates for subsequent explosion.

These results are challenged by my simulations of a 0.625-0.65 Msolar merger using the moving-mesh code AREPO. Unlike in GASOLINE, the merger remnant in AREPO not only has a relatively cold core, but one that is crescent-shaped and launches a one-armed spiral wave into its surroundings. I also insert weak magnetic fields into the WDs in AREPO, and find exponential field growth during their merger, leading to a > 10^10 G field within the remnant. Further study is required to understand how these novel features alter post-merger evolution.

Lastly, I calculate the evolution of idealized CO WDs experiencing runaway nuclear burning in their centers, which ends either with an explosion or expansion into a carbon-burning star. I determine the minimum mass for an explosion to be Mcrit ≈ 1.15 Msolar, which can be reached by the dense cores of some sub-Mch merger remnants. These remnants, however, are likely too underdense to explode, leaving only mergers with masses > Mch that can.

Reproduced with permission. library@astro.utoronto.ca
September 29, 2016