THERMONUCLEAR RUNAWAYS IN NOVA OUTBURSTS .2. EFFECT OF STRONG, INSTANTANEOUS, LOCAL FLUCTUATIONS

In an attempt to understand the manner in which nova outbursts are ini tiated on the surface of a white dwarf, we investigate the effects flu ctuations have on the evolution of a thermonuclear runaway. Fluctuatio ns in temperature, density, or the composition of material in the burn ing shell may arise due to the chaotic flow field generated by convect ion when it occurs, or by the accretion process itself. With the aid o f two-dimensional reactive flow calculations, we consider cases where a strong fluctuation in temperature arises during the early, quiescent accretion phase or during the later, more dynamic, explosion phase. I n all cases we find that an instantaneous, local temperature fluctuati on causes the affected material to become Rayleigh-Taylor unstable. Th e rapid rise and subsequent expansion of matter immediately cools the hot blob. which prevents the lateral propagation of burning. This sugg ests that local temperature fluctuations do not play a significant rol e in directly initiating the runaway, especially during the early stag es. However, they may provide an efficient mechanism of mixing core ma terial into the envelope (thereby pre-enriching the fuel for subsequen t episodes of explosive hydrogen burning) and of mixing substantial am ounts of the radioactive nucleus N-13 into the surface layers, making novae potential gamma-ray sources. This suggests that it is the global , not the local, evolution of the core-envelope interface to high temp eratures which dominates the development of the runaway. We also prese nt a possible new scenario for the initiation of nova outbursts based on our results.