Reactive flow in nova outbursts

Thermonuclear burning in hydrogen envelopes is studied using two-dimensional simulations, in a broader manner than that presented recently by Glasner & Livne. The evolution of the convective burning process before and during thermonuclear runaway (TNR) is discussed. We study the coupling between the burning and the hydrodynamics, which determines the nature of the convective flow, and analyze the relevant timescales. The consequences of the combustion and the nucleosynthesis are investigated in detail. The combustion process is composed of many local eruptions, scattered over the entire hydrogen envelope (at its base). Several mechanisms regulate the process in such a way that the TNR occurs over a time interval of several hundred seconds. These mechanisms are pressure waves, buoyancy, and turbulent diffusion. The TNR is terminated by a nearly spherically symmetric expansion of the outer layers of the envelope. For a 1 M_⊙ core, the enrichment of the hydrogen envelope by convective over-shoot is approximately 30%, in agreement with observations of novae.