INCOMPLETE CARBON-OXYGEN DETONATION IN TYPE Ia SUPERNOVAE

Incomplete carbon-oxygen detonation with reactions terminating after burning of C-12 in the leading C-12 + C-12 reaction (C-detonation) may occur in the low-density outer layers of white dwarfs exploding as Type Ia supernovae (SNe Ia). Previous studies of carbon-oxygen detonation structure and stability at low densities were performed under the assumption that the velocity of a detonation wave is derived from complete burning of carbon and oxygen to iron. In fact, at densities rho <= 10(6) g cm(-3) the detonation in SNe Ia may release less than a half of the available nuclear energy. In this paper, we study basic properties of such detonations. We find that the length of an unsupported steady-state C-detonation is similar or equal to 30-100 times greater than previously estimated and that the decreased energy has a drastic effect on the detonation stability. In contrast to complete detonations which are one-dimensionally stable, C-detonations may be one-dimensionally unstable and propagate by periodically re-igniting themselves via spontaneous burning. The re-ignition period at rho <= 10(6) g cm(-3) is estimated to be greater than the timescale of an SN Ia explosion. This suggests that propagation and quenching of C-detonations at these densities could be affected by the instability. Potential observational implications of this effect are discussed.