Neutron star-white dwarf mergers: Early evolution, physical properties, and outcomes

Neutron star (NS)-white dwarf (WD)mergersmay give rise to observable explosive transients, but have been little explored.We use 2D coupled hydrodynamical-thermonuclear FLASH-code simulations to study the evolution of WD debris discs formed following WD disruptions by NSs. We use a 19-element nuclear network and a detailed equation of state to follow the evolution, complemented by a post-process analysis using a larger 125-isotope nuclear network. We consider a wide range of initial conditions and study the dependence of the results on the NS/WD masses (1.4-2 and 0.375-0.7Mθ, respectively), WD composition (CO/He/hybrid He-CO), and the accretion-disc structure. We find that viscous inflow in the disc gives rise to continuous wind outflow of mostly C/O material mixed with nuclear-burning products arising from a weak detonation occurring in the inner region of the disc. We find that such transients are energetically weak (1048-1049 erg) compared with thermonuclear supernovae (SNe), and are dominated by the (gravitational) accretion energy. Although thermonuclear detonations occur robustly in all of our simulations (besides the He WD), they produce only little energy (1-10 per cent of the kinetic energy) and 56Ni ejecta (few ×10^-4-10^-3Mθ), with overall low ejecta masses of ∼0.01-0.1Mθ. Such explosions may produce rapidly evolving transients, much shorter and fainter than regular type Ia SNe. The composition and demographics of such SNe appear to be inconsistent with those of Ca-rich type Ib SNe. Though they might be related to the various classes of rapidly evolving SNe observed in recent years, they are likely to be fainter than the typical ones, and may therefore give rise to a different class of potentially observable transients.