Mixing Before and During Outburst in Classical and Symbiotic Novae

Abstract

An outine is given of the physical processes which are involved in the development of the abundance distributions found in classical and symbiotic novae. A helium buffer layer at the surface of the accreting white dwarf in the precursor binary system prevents the mixing between white dwarf matter and hydrogen-rich accreted matter until the mass of the buffer layer becomes large enough for the ignition of helium. The time required for this to occur is ~ 2-6 x 10⁶ years, longer than the lifetime of a symbiotic star, and this accounts for the absence of heavy element enhancements in symbiotic novae. Mixing between buffer-layer helium and accreted matter does, however, lead to large enhancements in the abundance of helium during the symbiotic nova phase. The cataclysmic variable precursors of classical novae live long enough for a helium shell flash to be ignited, and, if the flash is violent enough to remove the buffer layer entirely, large overabundances of heavy elements characterize subsequent nova outbursts. Detailed comparisons with observed abundances in nova eject a suggest that mixing by particle diffusion and by preoutburst convection must be supplemented by rotation-induced mixing.