Novae: The evolution from onset of convection to the runaway

All previous multidimensional calculations of nova thermonuclear runaways (TNR) were only able to simulate stages for which the relevant timescales were very short (seconds). Therefore, only the last phases of the ignition stage and the runaway itself have been studied in multiple dimensions. Improvements made in the hydro solver, and better computational resources, enable us to substantially extend previous research. We are almost able to resolve scales that are already unstable to the shear Kelvin-Helmholtz (KH) instability, thus improving the credibility of the results concerning undershoot mixing. Evolving the models in two dimensions from various early stages of the one-dimensional (1D) evolution puts our findings about the multidimensional effects on much firmer ground, since we can start our study closer to the stage at which the model is still stable against convection. For very early stages, the conditions of spherical symmetry and a jump in the composition, which are part of our assumptions for the 1D initial model, are legitimate. At early stages, we can also examine the fate of local perturbations related to the convective flow. A major issue for research is the ability of such early perturbations to ignite a flame that engulfs the whole envelope as an advancing burning front. Our limited experience with artificial parametric perturbations, presented here, denies this possibility. A major part of the research is devoted to close examinations of numerical effects that can compete with physical mechanisms. We try to estimate the uncertainty limits on the results, mainly on mixing, due to numerical issues. The timescales considered range from a phase close to the onset of convection, when the temperature at the base of the envelope is about 5 x 10(7) K, to the runaway itself.