We investigate the ejecta from black hole - neutron star mergers by modeling
the formation and interaction of mass ejected in a tidal tail and a disk wind.
The outflows are neutron-rich, giving rise to optical/infrared emission powered
by the radioactive decay of $r$-process elements (a kilonova). Here we perform
an end-to-end study of this phenomenon, where we start from the output of a
fully-relativistic merger simulation, calculate the post-merger hydrodynamical
evolution of the ejecta and disk winds including neutrino physics, determine
the final nucleosynthetic yields using post-processing nuclear reaction network
calculations, and compute the kilonova emission with a radiative transfer code.
We study the effects of the tail-to-disk mass ratio by scaling the tail
density. A larger initial tail mass results in fallback matter becoming mixed
into the disk and ejected in the subsequent disk wind. Relative to the case of
a disk without dynamical ejecta, the combined outflow has lower mean electron
fraction, faster speed, larger total mass, and larger absolute mass free of
high-opacity Lanthanides or Actinides. In most cases, the nucleosynthetic yield
is dominated by the heavy $r$-process contribution from the unbound part of the
tidal tail. A Solar-like abundance distribution can however be obtained when
the total mass of the dynamical ejecta is comparable to the mass of the disk
outflows. The kilonova has a characteristic duration of 1 week and a luminosity
of ~$10^{41}$ erg/s, with orientation effects leading to variations of a factor
~2 in brightness. At early times (< 1 day) the emission includes an optical
component from the (hot) Lanthanide-rich material, but the spectrum evolves
quickly to the infrared thereafter.
