Diffusion and mixing in accreting white dwarfs

Results are presented of numerical experiments conducted to determine the degree of enhancement of CNO elements in the envelope of a 1 solar mass carbon-oxygen white dwarf accreting hydrogen-rich material at rates of 10 exp -10, 10 exp -9, and 10 exp -8 solar mass/yr. In the steady state cases for which dM/dt is not less than 10 exp -9 solar mass/yr, both ordinary convection and diffusion contribute to the intermixing of hydrogen with carbon and oxygen prior to hydrogen ignition. The large contrast in radiative opacity near the interface between carbon-rich material and hydrogen-rich material is responsible for the establishment of a prerunaway convective zone. In the case of a helium buffer layer of 0.001 solar mass, approximately 3 percent of the mass of the helium layer is incorporated into the convective shell during the runaway, and negligible surface enhancements of heavy elements are found.