  Recent observations have revealed that the remnants of stellar-coalescence
transients are bipolar. This raises the questions of how these bipolar
morphologies arise and what they teach us about the mechanisms of mass ejection
during stellar mergers and common-envelope phases. In this paper, we analyze
hydrodynamic simulations of the lead-in to binary coalescence, a phase of
unstable Roche lobe overflow that takes the binary from the Roche limit
separation to the engulfment of the more compact accretor within the envelope
of the extended donor. As mass transfer runs away at increasing rates, gas
trails away from the binary. Contrary to previous expectations, early mass loss
from the system remains bound to the binary and forms a circumbinary torus.
Later ejecta, generated as the accretor grazes the surface of the donor, have
very different morphologies and are unbound. These two components of mass loss
from the binary interact as later, higher-velocity ejecta collide with the
circumbinary torus formed by earlier mass loss. Unbound ejecta are redirected
toward the poles, and escaping material creates a bipolar outflow. Our findings
show that the transition from bound to unbound ejecta from coalescing binaries
can explain the bipolar nature of their remnants, with implications for our
understanding of the origin of bipolar remnants of stellar-coalescence
transients and, perhaps, some preplanetary nebulae.
