  Black hole - neutron star (BH-NS) mergers are a major target for ground-based
gravitational wave (GW) observatories. A merger can also produce an
electromagnetic counterpart (a kilonova) if it ejects neutron-rich matter that
assembles into heavy elements through r-process nucleosynthesis. We study the
kilonova signatures of the unbound dynamical ejecta of a BH-NS merger. We take
as our initial state the results from a numerical relativity simulation, and
then use a general relativistic hydrodynamics code to study the evolution of
the ejecta with parameterized r-process heating models. The unbound dynamical
ejecta is initially a flattened, directed tidal tail largely confined to a
plane. Heating from the r-process inflates the ejecta into a more spherical
shape and smooths its small-scale structure, though the ejecta retains its bulk
directed motion. We calculate the electromagnetic signatures using a 3D
radiative transfer code and a parameterized opacity model for lanthanide-rich
matter. The light curve varies with viewing angle due to two effects:
asphericity results in brighter emission for orientations with larger projected
areas, while Doppler boosting results in brighter emission for viewing angles
more aligned with the direction of bulk motion. For typical r-process heating
rates, the peak bolometric luminosity varies by a factor of $\sim 3$ with
orientation while the peak in the optical bands varies by $\sim 3$ magnitudes.
The spectrum is blue-shifted at viewing angles along the bulk motion, which
increases the $V$-band peak magnitude to $\sim -14$ despite the lanthanide-rich
composition.
