Hydrodynamical simulations of helium-ignited binary white dwarf mergers

Abstract: Type Ia supernovae (SNe Ia) are common luminous astrophysical transients. SNe Ia serve as distance indicators for measuring the expansion rate of the universe and play important roles in galactic nucleosynthesis. However, ambiguities persist regarding the nature of their stellar progenitors and explosion mechanisms. The recent discovery of \textit{Gaia} hypervelocity white dwarfs (WDs) has provided direct evidence in support of helium-ignited double degenerate SNe Ia. In this study, we investigate the outcomes of helium-ignited double-degenerate WD mergers by performing a set of 3D hydrodynamical simulations with two different codes: \texttt{AREPO} and \texttt{FLASH}. We consider two distinct binary WD systems close to helium ignition, evolving each with both codes while keeping initial conditions fixed. The first binary WD model produces a double detonation of the primary WD and the hypervelocity ejection of the surviving secondary, similar to the canonical dynamically driven double degenerate double detonation (D6) scenario. In the second model, the secondary also undergoes a core detonation, resulting in the complete disruption of both WDs. Notably, despite utilizing distinct numerical solvers, nuclear reaction networks, and mesh strategies, \texttt{AREPO} and \texttt{FLASH} produce broadly consistent outcomes for both sets of initial conditions. While the nucleosynthetic yields differ due to the different nuclear reaction networks employed, the overall agreement between the simulations demonstrates the robustness of the numerical modeling of this scenario. Our results strongly support the viability of both the D6 and quadruple detonation channels for at least some SNe Ia. We explore the prospective observational signatures of this channel, including in the X-rays using \textit{XRISM's} \textit{RESOLVE}.