Effect of diffusion on prenova evolution: CNO-enriched envelopes

We have investigated the effect of diffusion at the core-envelope boundary of a I.25 M/sub sun/ C-O white dwarf with a 10-5 M/sub sun/ H-rich envelope (with Z = 0.03), a typical nova progenitor. The diffusion equations, which include concentration, pressure, and thermal terms, were solved along with the evolution equations for a mixture of three gases: H, He, and a third component, representative of all CNO elements present. Several models with different initial luminosities were evolved until a thermonuclear runaway developed. At the core-envelope boundary, heavy elements diffused outward, resulting in an insignificant CNO enrichment at the base of the envelope. Similarly, but more significantly, small amounts of H diffused inward, leading to H ignition in a very high Z zone. Thus, a thermonuclear runaway occurred at some depth below the initial core-envelope interface. As in time the entire region above the burning peak became convectively unstable, large amounts of C-O-rich matter were mixed into the envelope, increasing the values of Z up to 0.27. The effect of diffusion was stronger in models with lower initial luminosity. Sedimentation of heavy elements near the surface of the white dwarf was found to be unimportant for time scales typical of novae. We discuss the relevance of these results to models of fast novae