  Luminous red nova transients, presumably from stellar coalescence, exhibit
long-term precursor emission over hundreds of binary orbits, leading to
impulsive outbursts with durations similar to a single orbital period. In an
effort to understand these signatures, we present and analyze a hydrodynamic
model of unstable mass transfer from a giant-star donor onto a more compact
accretor in a binary system. Our simulation begins with mass transfer at the
Roche limit separation and traces a phase of runaway decay leading up to the
plunge of the accretor within the envelope of the donor. We characterize the
fluxes of mass and angular momentum through the system and show that the
orbital evolution can be reconstructed from measurements of these quantities.
The morphology of outflow from the binary changes significantly as the binary
orbit tightens. At wide separations, a thin stream of relatively high-entropy
gas trails from the outer Lagrange points. As the orbit tightens, the orbital
motion desynchronizes from the donor's rotation, and low-entropy ejecta trace a
broad fan of largely ballistic trajectories. An order-of-magnitude increase in
mass ejection rate accompanies the plunge of the accretor with the envelope of
the donor. We argue that this transition marks the precursor-to-outburst
transition observed in stellar coalescence transients.
