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}.

Figures (4)

• Figure 1: Time evolution of the Double_Det model shown in mass density (a) and $^{4}\mathrm{He}-$density (b) fields. In each sub-figure, the top row displays the AREPO simulation and the bottom row displays the FLASH simulation at corresponding evolutionary stages. All slices are taken at the $z=0$ mid-plane of the simulation domain. The mass density field shows the density structure of the binary, and $^{4}\mathrm{He}-$density highlights helium depletion due nuclear burning. From left to right: The two columns show the early propagation and wrap-around of the helium detonation on the primary WD surface; The third column displays the core detonation initiated by shock convergence; The fourth column shows the complete disruption of the primary WD with the ejecta impacting the secondary WD.
• Figure 2: Time evolution of the Double_Det model in the temperature (a) and the temperature gradient magnitude (b) fields. As in Figure \ref{['fig:double_det_densities_evolution']}, the top and bottom rows compare AREPO and FLASH simulations at corresponding evolutionary stages. All slices are taken at the $z=0$ mid-plane. The temperature field highlights regions of active nuclear burning, while the temperature gradient magnitude captures detonation fronts and the intricate internal shock structure. From left to right: the first two columns show the early surface helium detonation and its wrap-around (with zoomed-in insets revealing internal shocks); the third column displays the core detonation initiated after shock convergence; the fourth column shows the complete disruption of the primary WD and the resulting ejecta impact on the secondary WD.
• Figure 3: Time evolution of the Quad_Det model shown in mass density (a) and $^{4}\mathrm{He}-$density (b) fields. In each sub-figure, the top row displays the AREPO simulation and the bottom row displays the FLASH simulation at corresponding evolutionary stages. Because helium ignition in FLASH occurs $\sim 2.8\text{ s}$ earlier than in AREPO, the rows are phase-matched to the onset of surface ignition rather than absolute simulation time. All slices are taken at the $z=0$ mid-plane of the simulation domain. The mass density field shows the density structure of the binary, and $^{4}\mathrm{He}-$density highlights helium depletion due to nuclear burning. From left to right: the first column shows the pre-detonation state approximately $1.4\text{ s}$ prior to surface helium ignition of the primary WD; the second column illustrates the lateral propagation of the helium detonation across the primary WD surface ($\sim 1.3\text{ s}$ post-ignition); the third column shows the ejecta from the exploded primary impacting the secondary WD ($\sim 2\text{ s}$ after primary core detonation), and igniting its surface helium detonation; the fourth column shows the initiation of the core detonation in the secondary WD, by the convergence of shocks.
• Figure 4: Time evolution of the Quad_Det model shown in temperature (a) and temperature gradient magnitude (b) fields. As in Figure 3, the AREPO (top row) and FLASH (bottom row) simulations are phase-matched to account for the $\sim 2.8\text{ s}$ ignition time offset between the simulations. The temperature field highlights regions with higher temperatures and nuclear burning, while the temperature gradient magnitude captures the structure of the detonation fronts and shock interactions. From left to right: the first column shows the pre-detonation state approximately $1.4\text{ s}$ prior to surface helium ignition of the primary WD; the second column highlights the lateral propagation of the helium detonation and the resulting internal shocks ($\sim 1.3\text{ s}$ post-ignition); the third column captures the ejecta from the exploded primary impacting the secondary WD ($\sim 2\text{ s}$ after primary core detonation), igniting helium on the surface of the secondary WD; the fourth column shows the shocks produced by helium detonation, converge near the core and the igniting the core detonation in the secondary WD (zoomed-in inset plots show the core detonation initiated by the shock convergence).