Hydrodynamical shear mixing in subsonic boundary layers and its role in the thermonuclear explosion of classical novae

The transition zone between the white dwarf (WD) envelope and a circumstellar accretion disk in classical novae, the boundary layer, is a region of strong dissipation and intense vorticity. In this strongly sheared layer, the hydrogen-rich accreted gas is expected to mix with the underlying WD outermost layers so the conditions for the onset of the thermonuclear runaway (TNR) in classical nova will be different from the the standard treatment of the onset and subsequent mixing. We applied the critical layer instability (CLI) to the boundary between a disk-accreted H/He zone and the C/O - or O/Ne - rich outer layers of a mass-accreting WD in a cataclysmic binary and then used the resulting structure as input to one-dimensional nuclear-hydrodynamic simulations of the nova outburst. We simulated the subsonic mixing process in two dimensions for conditions appropriate for the inner disk and a CO 0.8 solar mass and CO and ONe 1.25 solar mass WDs using the compressible hydrodynamics code PLUTO. The resulting compositional profile was then imported into the one-dimensional nuclear-hydrodynamics code SHIVA to simulate the triggering and growth rate for the TNR and subsequent envelope ejection. We find that the deep shear-driven mixing changes the triggering and development of the TNR. In particular, the time to reach peak temperature is significantly shorter, and the ejected mass and maximum velocity of the ejecta substantially greater, than the current treatment. The 7Li yield is reduced by about an order of magnitude relative to the current treatments.