Off‐Center Deflagrations in Chandrasekhar Mass Type Ia Supernova Models

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Abstract
A series of two-dimensional numerical simulations of explosive nuclear burning is presented for white dwarfs near the Chandrasekhar mass. We assume that the burning begins as a slow deflagration front at or near the center of the star and continues until the density in the burning regions has declined to about 107 g cm–3, where the flame is essentially extinguished. We employ a novel numerical representation of the turbulent flame brush based upon ideas previously developed for modeling laboratory combustion and explore in some detail the sensitivity of the outcome to the manner in which burning is initiated. In particular, we simulate (1) a centrally ignited deflagration, (2) off-center ignition at a single "point," and (3) simultaneous off-center ignition at five "points." We find that the amount of 56Ni that is produced and other observable properties depend sensitively upon how the fuel is ignited. Because of the immediate onset of buoyant acceleration, the burning region in models ignited off center rises toward the surface more quickly than in the (commonly assumed) case of central ignition. With the exception of the model that ignited off-center at a single point, all models are unbound at the end of the computations, and between 0.59 M☉ (central ignition) and 0.65 M☉ (ignition at multiple "points") of matter are processed into nuclear-burning products. These results would guarantee an observable, though weak, Type Ia supernova. Our results are expected to change for simulations in three dimensions, especially for the off-center ignitions discussed in this paper, and late detonations driven by pulsations are not unambiguously excluded. We can, however, state that the chances for a direct transition to a detonation appear small because, in all our models, the turbulent velocity of the burning front remains very subsonic.
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