Surface Detonations in Double Degenerate Binary Systems Triggered by Accretion Stream Instabilities

TL;DR

This paper uses 3D simulations to explore how accretion stream instabilities can trigger surface detonations in white dwarf binaries, potentially leading to supernova explosions.

Contribution

It introduces a new mechanism where Kelvin-Helmholtz instabilities from accretion streams induce helium detonations on white dwarfs, a novel pathway for sub-Chandrasekhar supernovae.

Findings

Kelvin-Helmholtz instabilities seed dense knots impacting the surface.

Surface helium detonation can trigger CO core detonation.

Accretion stream dynamics critically influence detonation conditions.

Abstract

We present three-dimensional simulations on a new mechanism for the detonation of a sub-Chandrasekhar CO white dwarf in a dynamically unstable system where the secondary is either a pure He white dwarf or a He/CO hybrid. For dynamically unstable systems where the accretion stream directly impacts the surface of the primary, the final tens of orbits can have mass accretion rates that range from $10^{-5}$ to $10^{-3} M_{\odot}$ s$^{-1}$, leading to the rapid accumulation of helium on the surface of the primary. After $\sim 10^{-2}$ $M_{\odot}$ of helium has been accreted, the ram pressure of the hot helium torus can deflect the accretion stream such that the stream no longer directly impacts the surface. The velocity difference between the stream and the torus produces shearing which seeds large-scale Kelvin-Helmholtz instabilities along the interface between the two regions. These instabilities eventually grow into dense knots of material that periodically strike the surface of the primary, adiabatically compressing the underlying helium torus. If the temperature of the compressed material is raised above a critical temperature, the timescale for triple-$\alpha$ reactions becomes comparable to the dynamical timescale, leading to the detonation of the primary's helium envelope. This detonation drives shockwaves into the primary which tend to concentrate at one or more focal points within the primary's CO core. If a relatively small amount of mass is raised above a critical temperature and density at these focal points, the CO core may itself be detonated.