On Heavy Element Enrichment in Classical Novae

Many classical nova ejecta are enriched in CNO and Ne. Rosner et al. recentlysuggested that the enrichment might originate in the resonant interactionbetween large-scale shear flows in the accreted H/He envelope and gravity wavesat the interface between the envelope and the underlying C/O white dwarf. Theshear flow amplifies the waves, which eventually form cusps and break. Thiswave breaking injects a spray of C/O into the superincumbent H/He. Usingtwo-dimensional simulations, we formulate a quantitative expression for theamount of C/O per unit area that can be entrained into the H/He at saturation.The fraction of the envelope that is enriched depends on the horizontaldistribution of shear velocity and the density contrast between the C/O whitedwarf and the H/He layer but is roughly independent of the vertical shape ofthe shear profile. Using this parameterization for the mixed mass, we thenperform several one-dimensional Lagrangian calculations of an accreting whitedwarf envelope and consider two scenarios: that the wave breaking and mixing isdriven by the convective flows; and that the mixing occurs prior to the onsetof convection. In the absence of enrichment prior to ignition, the base of theconvective zone, as calculated from mixing-length theory with the Ledouxinstability criterion, does not reach the C/O interface. As a result, there isno additional mixing, and the runaway is slow. In contrast, the formation of amixed layer during the accretion of H/He, prior to ignition, causes a moreviolent runaway. The envelope can be enriched by <25% of C/O by mass(consistent with that observed in some ejecta) for shear velocities, over thesurface, with Mach numbers <0.4.Comment: 6 pages, 6 figures, uses emulateapj.cls (included); minor changes to text from v1, to appear in Ap