Annu. Rev. Astron. Astrophys. 1998. 36: 17-55 Copyright © 1998 by Annual Reviews. All rights reserved

4.2. Explosion Models

In the 1990s, a variety of explosion models other than the classical deflagration has been considered. They can be divided into "carbon ignitors" and "helium ignitors" according to whether the first nuclear ignition is of carbon, deep inside the accretor, or of helium near the surface.

Among the carbon-ignitor models, deflagrations, "delayed detonations" (Khokhlov et al 1993, Arnett & Livne 1994a, b, Woosley & Weaver 1994b, Höflich 1995, Nomoto et al 1997), and "pulsating delayed detonations" (Khokhlov et al 1993) could be applicable in the single-degenerate case, so these have been constructed for Chandrasekhar-mass ejection. In the double-degenerate case, the classical idea (see Iben 1997 for a review) is that the merger leads to a super-Chandrasekhar configuration consisting of a white-dwarf core, a quasispherical pressure-supported envelope, and a low-density thick disk. Whether this flying saucer (Iben & Tutukov 1984) eventually explodes or collapses is thought to depend on, among other things, whether carbon ignites at the core-disk boundary and burns steadily inward to produce an oxygen-neon-magnesium configuration that will simply collapse to a neutron star. A different posssibility is that explosion of one and then the other white dwarf occurs during or even just before the merger, owing to tidal or shear heating (Iben 1997). Shigayama et al (1992), Khokhlov et al (1993) constructed some spherically symmetric explosion models with super-Chandrasekhar merger products in mind.

In the 1990s, there has been much interest in sub-Chandrasekhar helium-ignitor models, as constructed by Woosley & Weaver (1994a), Livne & Arnett (1995), Höflich & Khokhkov (1996). The first nuclear ignition is near the bottom of a helium layer of about 0.2 M accumulated on top of a C-O white dwarf. A prompt detonation propagates outwards through the helium while an inward nonburning pressure wave compresses and ignites the underlying C-O (perhaps well off center) and drives a second detonation outwards through the C-O. Owing to the difference between the nuclear kinetics of carbon and helium burning, these models have a composition structure that is fundamentally different from that of carbon ignitors. ⁴He burns to ¹²C by the slow triple alpha process, and as soon as ¹²C is formed, it rapidly captures alpha particles to form ⁵⁶Ni, so the original helium layer ends up as a high-velocity mixture of ⁵⁶Ni and leftover ⁴He. In these models intermediate-mass elements such as silicon and sulfur, produced by low-density carbon burning, are ejected in a relatively narrow range of velocity around 10,000 km s^-1.