Thermonuclear explosion of a massive hybrid HeCO white dwarf triggered by a He detonation on a companion

Normal type Ia supernovae (SNe) are thought to arise from the thermonuclear explosion of massive (>0.8 Mpdbl) carbon-oxygen white dwarfs (WDs), although the exact mechanism is debated. In some models, helium accretion on to a carbon-oxygen (CO) WD from a companion was suggested to dynamically trigger a detonation of the accreted helium shell. The helium detonation then produces a shock that after converging on itself close to the core of the CO WD, triggers a secondary carbon detonation, and gives rise to an energetic explosion. However, most studies of such scenarios have been done in one or two dimensions, and/or did not consider self-consistent models for the accretion and the He donor. Here, we make use of detailed 3D simulation to study the interaction of a He-rich hybrid $0.69\, \mathrm{M_\odot }$ HeCO WD with a more massive $0.8\, \mathrm{M_\odot }$ CO WD. We find that accretion from the hybrid WD on to the CO WD gives rise to a helium detonation. However, the helium detonation does not trigger a carbon detonation in the CO WD. Instead, the helium detonation burns through the accretion stream to also burn the helium shell of the donor hybrid HeCO WD. The detonation of its massive helium shell then compresses its CO core, and triggers its detonation and full destruction. The explosion gives rise to a faint, likely highly reddened transient, potentially observable by the Vera Rubin survey, and the high-velocity ($\sim \! 1000\, \mathrm{km s{-1}}$) ejection of the heated surviving CO WD companion. Pending on uncertainties in stellar evolution, we estimate the rate of such transient to be up to $\sim \! 10{{\ \rm per\ cent}}$ of the rate of type Ia SNe.