SPH Simulations of Direct Impact Accretion in the Ultracompact AM CVn Binaries

The ultracompact binary systems V407 Vul (RX J1914.4+2456) and HM Cnc (RX J0806.3+1527), a two-member subclass of the AM CVn stars, continue to generate interest because they defy unambiguous classification. Three proposed models remain viable at this time, but none of the three are significantly more compelling than the remaining two, and all three can satisfy the observational constraints if parameters in the models are tuned. One of the three proposed models is the direct impact model of Marsh & Steeghs, in which the accretion stream impacts the surface of a rapidly rotating primary white dwarf directly, but at a near-glancing angle. One requirement of this model is that the accretion stream have a high enough density to advect its specific kinetic energy below the photosphere for progressively more thermalized emission downstream, a constraint that requires an accretion spot size of ~ -->1.2 × 105 km2 or smaller. Because we had at hand a smoothed particle hydrodynamics code optimized for cataclysmic variable accretion disk simulations, it was relatively straightforward for us to adapt it to calculate the footprint of the accretion stream at the nominal radius of the primary white dwarf and thus to test this constraint of the direct impact model. We find that the mass flux at the impact spot can be approximated by a bivariate Gaussian with a standard deviation of -->σ = 164 km in the orbital plane and -->σθ = 23 km in the perpendicular direction. The area of the 2 σ ellipse into which ~86% of the mass flux occurs is roughly 47,400 km2, or roughly half the size estimated by Marsh & Steeghs. We discuss the necessary parameters of a simple model of the luminosity distribution in the postimpact emission region.