Novae: The Evolution from Onset of Convection to the Runaway

Abstract

All previous multidimensional calculations of nova thermonuclearrunaways (TNR) were only able to simulate stages for which therelevant timescales were very short (seconds). Therefore, only thelast phases of the ignition stage and the runaway itself have beenstudied in multiple dimensions. Improvements made in the hydrosolver, and better computational resources, enable us to substantiallyextend previous research. We are almost able to resolve scales thatare already unstable to the shear Kelvin-Helmholtz (KH) instability,thus improving the credibility of the results concerning undershootmixing. Evolving the models in two dimensions from various earlystages of the one-dimensional (1D) evolution puts our findings aboutthe multidimensional effects on much firmer ground, since we canstart our study closer to the stage at which the model is stillstable against convection. For very early stages, the conditionsof spherical symmetry and a jump in the composition, which are partof our assumptions for the 1D initial model, are legitimate. Atearly stages, we can also examine the fate of local perturbationsrelated to the convective flow. A major issue for research is theability of such early perturbations to ignite a flame that engulfsthe whole envelope as an advancing burning front. Our limitedexperience with artificial parametric perturbations, presented here,denies this possibility. A major part of the research is devotedto close examinations of numerical effects that can compete withphysical mechanisms. We try to estimate the uncertainty limits onthe results, mainly on mixing, due to numerical issues. The timescalesconsidered range from a phase close to the onset of convection,when the temperature at the base of the envelope is about 5?107 K,to the runaway itself.