The Viscous Evolution of White Dwarf Merger Remnants

TL;DR

This paper uses multi-dimensional hydrodynamic simulations to study how white dwarf merger remnants evolve under viscous forces, revealing potential pathways to supernovae and other astrophysical phenomena.

Contribution

It introduces detailed viscous evolution models of WD merger remnants, highlighting their sphericalization, minimal mass loss, and possible ignition of detonations in He envelopes.

Findings

Remnants become spherical within hours despite initial rotation.

Less than 10^{-5} solar masses are unbound during evolution.

Viscous heating can trigger He detonation, leading to potential Type Ia supernovae.

Abstract

The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multi-dimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an $\alpha$-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al. (2011), which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towards spherical states on time-scales of hours, even though a significant fraction of the mass is initially rotationally supported. The viscous evolution unbinds only a very small amount of mass $(< 10^{-5} M_\odot)$. Viscous heating causes some of the systems we study with He WD secondaries to reach conditions of nearly dynamical burning. It is thus possible that the post-merger viscous phase triggers detonation of the He envelope in some WD mergers, potentially producing a Type Ia supernova via a double detonation scenario. Our calculations provide the proper initial conditions for studying the long-term thermal evolution of WD merger remnants. This is important for understanding WD mergers as progenitors of Type Ia supernovae, neutron stars, R Coronae Borealis stars and other phenomena.