On Heavy Element Enrichment in Classical Novae
Abstract
Many classical nova ejecta are enriched in CNO and Ne. Rosner et al. recently suggested that the enrichment might originate in the resonant interaction between large-scale shear flows in the accreted H/He envelope and gravity waves at the interface between the envelope and the underlying C/O white dwarf. The shear flow amplifies the waves, which eventually form cusps and break. This wave breaking injects a spray of C/O into the superincumbent H/He. Using two-dimensional simulations, we formulate a quantitative expression for the amount of C/O that can be entrained into the H/He at saturation. The fraction of the envelope that is enriched depends on the shear velocity and density contrast between the C/O white dwarf and the H/He layer but is roughly independent of the shape of the shear profile. Using this parameterization for the mixed mass, we then perform several one-dimensional Lagrangian calculations of an accreting white dwarf envelope and consider two scenarios: that the wave breaking and mixing is driven by the convective flows; and that the mixing occurs prior to the onset of convection. In the absence of enrichment prior to ignition, the base of the convective zone, as calculated from mixing-length theory, does not reach the C/O interface. As a result, there is no additional mixing, and the nova is slow. In contrast, the formation of a mixed layer during the accretion of H/He, prior to ignition, causes a more violent runaway. For large shear velocities (Mach numbers > 0.3) the envelope can be enriched to > 20% of C/O by mass, consistent with that observed in some ejecta.Keywords
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