The Evolution and Fate of Super-Chandrasekhar Mass White Dwarf Merger Remnants

TL;DR

This study models the evolution of super-Chandrasekhar mass white dwarf merger remnants, revealing they do not undergo collapse but may form neutron stars or massive white dwarfs, with observable implications.

Contribution

It provides detailed stellar evolution calculations showing the fate of massive white dwarf merger remnants, challenging previous assumptions about their collapse.

Findings

Remnants appear as luminous, dusty sources similar to extreme AGB stars.

Off-center carbon fusion propagates inward, transforming the WD composition.

Remnants with mass ≥ 1.35 M_sun ignite neon, leading to silicon WD formation.

Abstract

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 $L \sim 3 \times 10^4 L_\odot$ source for $\sim 10^4$ yr. A dusty wind may develop, leading these sources to be self-obscured and to appear similar to extreme AGB stars. Roughly $\sim 10$ such objects should exist in the Milky Way and M31 at any time. As found in previous work, off-center carbon fusion is ignited within the merger remnant and propagates inward 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 center, neutrino-cooled Kelvin-Helmholtz contraction leads to off-center neon ignition in remnants with masses $\ge 1.35 M_\odot$. 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.