The origin of hypervelocity white dwarfs in the merger-disruption of He-CO white dwarfs

TL;DR

This paper presents a hydrodynamic simulation showing that the merger of two hybrid white dwarfs can produce hypervelocity white dwarfs traveling at speeds over 2000 km/s, explaining their observed properties and suggesting a new formation channel.

Contribution

It introduces a novel merger scenario involving hybrid white dwarfs that accounts for hypervelocity white dwarfs and their characteristics, expanding understanding of their origins.

Findings

Merger causes partial disruption and double-detonation explosion.

Remnant core ejected at ~2000 km/s matches observed HVWD speeds.

Explains luminosities and temperatures of hot HVWDs.

Abstract

Hypervelocity white dwarfs (HVWDs) are stellar remnants moving at speeds exceeding the Milky Way's escape velocity. The origins of the fastest HVWDs are enigmatic, with proposed formation scenarios facing challenges explaining both their extreme velocities and observed properties. Here we report a three-dimensional hydrodynamic simulation of a merger between two hybrid Helium-Carbon-Oxygen white dwarfs (HeCO WDs with masses of 0.69 and 0.62 M$_\odot$). We find that the merger leads to a partial disruption of the secondary WD, coupled with a double-detonation explosion of the primary WD. This launches the remnant core of the secondary WD at a speed of $~2000$ km s$^{-1}$, consistent with observed HVWDs. The low mass of the ejected remnant and its heating from the primary WD's ejecta explain the observed luminosities and temperatures of hot HVWDs, which are otherwise difficult to reconcile with previous models (such as the D6). This discovery establishes a new formation channel for HVWDs and points to a previously unrecognized pathway for producing peculiar Type Ia supernovae and faint explosive transients.