  Neutron-star (NS) - white-dwarf (WD) mergers may 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-disks formed following WD-disruptions by NSs. 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 ($1.4-2{\rm M_{\odot}}$;$\,{\rm
0.375-0.7\,M_{\odot}}$, respectively), WD-composition (CO/He/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
($10^{48}-10^{49}$ergs) 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
$^{56}{\rm Ni}$ ejecta (few$\times10^{-4}-10^{-3}{\rm M_{\odot}})$, with
overall low ejecta masses of $\sim0.01-0.1{\rm M_{\odot}}$. 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 a
different class of potentially observable transients.
