Fires and Floods on White Dwarfs: A Nova Model

Alan C. Calder (U. of Chicago)

Classical nova explosions occur in binary systems composed of a main sequence star and a white dwarf. Hydrogen-rich material from the main sequence companion accretes onto the surface of the white dwarf until the density and temperature are high enough at the base of the accreted layer to ignite the material. The result is a thermonuclear runaway that produces an observable outburst. Calculations have shown that this outburst cannot occur unless there is mixing of intermediate-mass elements from the white dwarf into the accreted material. I will describe recent studies of novae and the mixing mechanism performed at the ASC Flash Center at the University of Chicago. These include quantitative results from simulations of breaking resonant gravity waves driven by a "wind" of accreted material, which yield a well-mixed layer, and one-dimensional simulations of nova explosions using the results of the gravity wave simulations as a sub-grid enrichment model. Our results indicate that breaking gravity waves mechanism are a robust mechanism that can provide up to 20% enrichment by mass of the envelope, which is sufficient to produce vigorous explosions.