3D Hydrodynamical Simulations of Helium-Ignited Double-degenerate White Dwarf Mergers

TL;DR

This study uses 3D hydrodynamical simulations to investigate helium-ignited double-degenerate white dwarf mergers, revealing that helium detonation often fails to ignite the core, resulting in faint transients instead of typical supernovae.

Contribution

First 3D self-consistent modeling of helium detonation in double white dwarf mergers near Roche lobe overflow, exploring conditions leading to supernovae or faint transients.

Findings

Helium layer detonates without core ignition, causing faint nova-like transients.

Failed core detonation suggests SNe Ia may only occur in very massive CO WDs.

Radioisotope $^{44}$Ti production could indicate the nature of the explosion.

Abstract

The origins of type Ia supernovae (SNe Ia) are still debated. Some of the leading scenarios involve a double detonation in double white dwarf (WD) systems. In these scenarios, helium shell detonation occurs on top of a carbon-oxygen (CO) WD, which then drives the detonation of the CO-core, producing a SN Ia. Extensive studies have been done on the possibility of a double helium detonation, following a dynamical helium mass-transfer phase onto a CO-WD. However, 3D self-consistent modeling of the double-WD system, the mass transfer, and the helium shell detonation have been little studied. Here we use 3D hydrodynamical simulations to explore this case in which a helium detonation occurs near the point of Roche lobe overflow of the donor WD and may lead to an SN Ia through the dynamically driven double-degenerate double-detonation (D6) mechanism. We find that the helium layer of the accreting primary WD does undergo a detonation, while the underlying carbon-oxygen core does not, leading to an extremely rapid and faint nova-like transient instead of a luminous SN Ia event. This failed core detonation suggests that D6 SNe Ia may be restricted to the most massive carbon-oxygen primary WDs. We highlight the nucleosynthesis of the long-lived radioisotope $^{44}$Ti during explosive helium burning, which may serve as a hallmark both of successful as well as failed D6 events which subsequently detonate as classical double-degenerate mergers.