Nuclear-dominated accretion and subluminous supernovae from the merger
  of a white dwarf with a neutron star or black hole
  We construct one dimensional steady-state models of accretion disks produced
by the tidal disruption of a white dwarf (WD) by a neutron star (NS) or stellar
mass black hole (BH). At radii r <~ 1e8.5-1e9 cm the midplane density and
temperature are sufficiently high to burn the initial white dwarf material into
increasingly heavier elements (e.g. Mg, Si, S, Ca, Fe, and Ni) at sequentially
smaller radii. When the energy released by nuclear reactions is comparable to
that released gravitationally, we term the disk a nuclear-dominated accretion
flow (NuDAF). At small radii <~1e7 cm Fe photo-disintegrates into He and then
free nuclei, and cooling by neutrinos may be efficient. At the high accretion
rates of relevance ~ 0.1-1e-4 Msun/s, most of the disk is radiatively
inefficient and prone to outflows powered by viscous dissipation and nuclear
burning. Outflow properties are calculated by requiring that material in the
midplane be marginally bound (Bernoulli constant <~ 0), due (in part) to
cooling by matter escaping the disk. For reasonable assumptions regarding the
properties of disk winds, we show that a significant fraction >50-80 per cent
of the total WD mass is unbound. The ejecta composition is predominantly O, C,
Si, Mg, Ne, Fe, and S [He, C, Si, S, Ar, and Fe], in the case of C-O [He] WDs,
respectively, along with a small quantity ~1e-3-1e-2 Msun of radioactive Ni56
and, potentially, a trace amount of H. We use our results to evaluate possible
EM counterparts of WD-NS/BH mergers, including optical transients powered by
the radioactive decay of Ni56 and radio transients powered by the interaction
of the ejecta with the interstellar medium. We address whether recently
discovered subluminous Type I supernovae result from WD-NS/BH mergers. Our
results also have implications for accretion following the core collapse of
massive stars in collapsar models for gamma-ray bursts.
