We study the violent phase of the merger of massive binary white dwarf systems. Our aim is to characterize the conditions for explosive burning to occur, and identify a possible explosion mechanism of Type Ia supernovae. The primary components of our model systems are carbon-oxygen (C/O) white dwarfs, while the secondaries are made either of C/O or of pure helium. We account for tidal effects in the initial conditions in a self-consistent way, and consider initially well-separated systems with slow inspiral rates. We study the merger evolution using an adaptive mesh refinement, reactive, Eulerian code in three dimensions, assuming symmetry across the orbital plane. We use a co-rotating reference frame to minimize the effects of numerical diffusion, and solve for self-gravity using a multi-grid approach. We find a novel detonation mechanism in C/O mergers with massive primaries. Here the detonation occurs in the primarys core and relies on the combined action of tidal heating, accretion heating, and self-heating due to nuclear burning. The exploding structure is compositionally stratified, with a reverse shock formed at the surface of the dense ejecta. The existence of such a shock has not been reported elsewhere. The explosion energy ($1.6times 10^{51}$ erg) and $^{56}$Ni mass (0.86 M$_odot$) are consistent with a SN Ia at the bright end of the luminosity distribution, with an approximated decline rate of $Delta m_{15}(B)approx 0.99$. Our study does not support double-detonation scenarios in the case of a system with a 0.6 M$_odot$ helium secondary and a 0.9 M$_odot$ primary. Although the accreted helium detonates, it fails to ignite carbon at the base of the boundary layer or in the primarys core.
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