We study transients produced by equatorial disk-like outflows from catastrophically mass-losing binary stars with an asymptotic velocity and energy deposition rate near the inner edge which are proportional to the binary escape velocity v _ { esc } .
As a test case , we present the first smoothed-particle radiation-hydrodynamics calculations of the mass loss from the outer Lagrange point with realistic equation of state and opacities .
The resulting spiral stream becomes unbound for binary mass ratios 0.06 \lesssim q \lesssim 0.8 .
For synchronous binaries with non-degenerate components , the spiral-stream arms merge at a radius of \sim 10 a , where a is the binary semi-major axis , and the accompanying shock thermalizes about 10 \% of the kinetic power of the outflow .
The mass-losing binary outflows produce luminosities reaching up to \sim 10 ^ { 6 } L _ { \odot } and effective temperatures spanning 500 \lesssim T _ { eff } \lesssim 6000 K , which is compatible with many of the class of recently-discovered red transients such as V838 Mon and V1309 Sco .
Dust readily forms in the outflow , potentially in a catastrophic global cooling transition .
The appearance of the transient is viewing angle-dependent due to vastly different optical depths parallel and perpendicular to the binary plane .
We predict a correlation between the peak luminosity and the outflow velocity , which is roughly obeyed by the known red transients .
Outflows from mass-losing binaries can produce luminous ( 10 ^ { 5 } L _ { \odot } ) and cool ( T _ { eff } \lesssim 1500 K ) transients lasting a year or longer , as has potentially been detected by Spitzer surveys of nearby galaxies .