DIFFUSION AND MIXING IN CLASSICAL NOVA PRECURSORS

Numerical experiments have been conducted to determine the degree of enhancement of CNO elements in the envelope of an initially cold 1 M. carbon-oxygen white dwarf accreting hydrogen-rich material at rates expected in cataclysmic variables. Only ordinary particle diffusion and convective mixing have been considered, and the enhancements found are a consequence of the diffusive intermixing of hydrogen with carbon and oxygen; this mixing causes a hydrogen-burning thermonuclear runaway to begin below the surface of the original white dwarf. Convection mixes matter between the ignition point and the surface. Enhancements (relative to solar) vary from a factor of 10.5 at dM/dt = 10(-8) M. yr-1 to a factor of 26.6 at dM/dt = 10(-10) M. yr-1. This demonstrates that, if more mass is lost in a nova explosion than is gained between nova outbursts (ensuring that a large buffer layer composed of helium does not grow and prevent intermixing of hydrogen and carbon and oxygen), no mixing mechanisms other than simple diffusion and convection need be invoked to account for the large enhancements of CNO elements observed in the spectra of many classical novae.