CONSTRAINTS ON ELEMENT MIXING IN CLASSICAL NOVAE

The ejecta of many classical novae are enriched, relative to hydrogen, either in helium, or in CNO and other heavy elements, or in helium as well as in heavier elements. Building on our current theoretical understanding of hydrogen shell flashes, we propose a model which takes into account the effect of element mixing between interior and accreted layers and allows us to predict the abundances of nova ejecta as a function of the assumed degree of mixing. The model contains two parameters. The first is a measure of the degree of element mixing, and the second is a measure of the mass M(acc) accreted between nova outbursts. The two parameters are determined as a function of assumed white dwarf mass ff the abundances of helium and of heavy elements relative to hydrogen can be specified from the observations. Requiring our model to fit abundances estimated for matter in 11 nova ejecta, we determine the degree of mixing, place limits on the mass of the underlying white dwarf, and estimate M(acc). In every case, the required degree of mixing is (often much) larger than that predicted by models in which only particle diffusion and preoutburst convection have been taken into account. Estimated values of M(acc) vary from approximately 10(-6) M. (corresponding to a theoretical accretion rate greater than 10(-8) M. yr-1) to approximately 5 x 10(-4) M. (corresponding to a theoretical accretion rate less than 10(-10) M. yr-1). Particle diffusion may play an important role in producing overabundances of heavy elements when M(acc) is large and accretion rate M is small (M less than or similar to 2 X 10(-10) M. yr-1). However, some form of rotationally induced mixing is essential for producing the observed heavy element abundances when M(acc) is small and M is large (M greater than or similar to 10(-9) M. yr-1), and an extramixing mechanism is also required for some novae when M(acc) is large and M is small.