The evolution and fate of super-Chandrasekhar mass white dwarf merger remnants

We present stellar evolution calculations of the remnant of the merger of two carbon-oxygen white dwarfs (CO WDs). We focus on cases that have a total mass in excess of the Chandrasekhar mass. After the merger, the remnant manifests as an |$L \sim 3\times 10^4\ \mathrm{\mathrm{L}_{\odot }}$| source for ∼104 yr. A dusty wind may develop, leading these sources to be self-obscured and to appear similar to extreme asymptotic giant branch (AGB) stars. Roughly ∼10 such objects should exist in the Milky Way and M31 at any time. As found in previous work, off-centre carbon fusion is ignited within the merger remnant and propagates inwards via a carbon flame, converting the WD to an oxygen-neon (ONe) composition. By following the evolution for longer than previous calculations, we demonstrate that after carbon-burning reaches the centre, neutrino-cooled Kelvin–Helmholtz contraction leads to off-centre neon ignition in remnants with masses |$\ge 1.35\ \mathrm{\mathrm{M}_{\odot }}$|⁠. The resulting neon-oxygen flame converts the core to a silicon WD. Thus, super-Chandrasekhar WD merger remnants do not undergo electron-capture induced collapse as traditionally assumed. Instead, if the remnant mass remains above the Chandrasekhar mass, we expect that it will form a low-mass iron core and collapse to form a neutron star. Remnants that lose sufficient mass will end up as massive, isolated ONe or Si WDs.

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