Nuclear-dominated accretion flows in two dimensions – II. Ejecta dynamics and nucleosynthesis for CO and ONe white dwarfs

We study mass ejection from accretion discs formed in the merger of a white dwarf with a neutron star or black hole. These discs are mostly radiatively inefficient and support nuclear fusion reactions, with ensuing outflows and electromagnetic transients. Here we perform time-dependent, axisymmetric hydrodynamic simulations of these discs including a physical equation of state, viscous angular momentum transport, a coupled 19-isotope nuclear network, and self-gravity. We find no detonations in any of the configurations studied. Our global models extend from the central object to radii much larger than the disc. We evolve these global models for several orbits, as well as alternate versions with an excised inner boundary to much longer times. We obtain robust outflows, with a broad velocity distribution in the range 10²–10⁴ km s⁻¹. The outflow composition is mostly that of the initial white dwarf, with burning products mixed in at the 10–30 per cent level by mass, including up to ∼10⁻² M of ⁵⁶Ni. These heavier elements (plus ⁴He) are ejected within 40^◦ of the rotation axis, and should have higher average velocities than the lighter elements that make up the white dwarf. These results are in broad agreement with previous one- and two-dimensional studies, and point to these systems as progenitors of rapidly rising (∼ few day) transients. If accretion on to the central BH/NS powers a relativistic jet, these events could be accompanied by high-energy transients with peak luminosities ∼10⁴⁷–10⁵⁰ erg s⁻¹ and peak durations of up to several minutes, possibly accounting for events like CDF-S XT2.