The origin of hypervelocity white dwarfs in the merger disruption of He–C–O white dwarfs

Abstract

Hypervelocity white dwarfs (HVWDs) are stellar remnants moving at speeds that exceed the Milky Way’s escape velocity. The origins of the fastest HVWDs are enigmatic, with proposed formation scenarios struggling to explain both their extreme velocities and observed properties. Here we report a three-dimensional hydrodynamic simulation of a merger between two hybrid helium–carbon–oxygen WDs (with masses of 0.69 M_⊙ and 0.62 M_⊙, where M_⊙ is the mass of the Sun). 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 2,000 km s⁻¹, consistent with observed HVWDs. The low mass of the ejected remnant and 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 dynamically driven double-degenerate double-detonation scenario). This discovery establishes a new formation channel for HVWDs and points to a pathway for producing peculiar type Ia supernovae and faint explosive transients.