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Nuclear Dominated Accretion Flows in Two Dimensions. I. Torus Evolution with Parametric Microphysics

We explore the evolution of radiatively inefficient accretion disks in which nuclear reactions are dynamically important (`Nuclear Dominated Accretion Flows', or NuDAFs). Examples of such disks are those generated by the merger of a white dwarf with a neutron star or black hole, or by the collapse of a rotating star. Here we present two-dimensional hydrodynamic simulations that systematically explore the effect of adding a single nuclear reaction to a viscous torus. The equation of state, anomalous shear stress, and nuclear reactions are given parametric forms. Our results point to the existence of two qualitatively different regimes of NuDAF evolution: (1) steady accretion with quiescent burning; or (2) detonation of the disk. These outcomes are controlled primarily by the ratio of the nuclear energy released to the enthalpy at the burning radius. Disks detonate if this ratio exceeds a critical value (~1), and if burning occurs in regions where neutrino cooling is unimportant. Thermonuclear runaways are seeded by the turbulent mixing of hot ash with cold fuel at the burning front. Subcritical disks do not explode, but instead power a persistent collimated outflow of unbound material composed primarily of ash, with a mass-loss rate that increases with the nuclear binding energy released. We discuss the implications of our results for supernova-like counterparts from astrophysical events in the NuDAF regime. In particular, detonations following a white dwarf - neutron star merger could account for some subluminous Type Ia supernovae, such as the class defined by SN 2002cx.