Special TAPIR Seminar

Neutron-star (NS) - white-dwarf (WD) mergers may give rise to observable explosive transients. However, the evolution and properties of such events have been little explored. We use 2D coupled hydrodynamical-thermonuclear FLASH-code simulations to study the evolution of WD debris-disks formed following the WD disruption by the NS. We use a 19-elements nuclear-network and a detailed equation of state to follow the evolution, complemented by a post-process analysis using a larger 125- isotopes nuclear-network. We consider a wide range of initial conditions and study the dependence of the results on the NS/WD masses (1.4−2M_⊙; 0.375 − 0.7 M_⊙, respectively), WD-composition (CØ/He or hybrid-He-CO) and the accretion-disk structure. We find that viscous inflow in the disk 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 disk. We find that such transients are energetically weak (10⁴⁸ − 10⁴⁹ergs) 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% of the kinetic energy) and ⁵⁶Ni ejecta (few ×10⁻⁴ − 10⁻³M_⊙). Most of the accretion occurs on the viscous time-scale, with little contribution on longer tens of seconds time-scale. The ejecta velocities and masses are low compared with type-Ia SNe (3000-6000 km s⁻¹;  a few × 0.1M_⊙). The properties of such events may produce rapidly evolving transients, but these are likely much shorter and much 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 a different class of transients possibly observable with future deep transient surveys.