2016MNRAS.455.4351P

We study transients produced by equatorial disc-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 vesc. 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 ≲ q ≲ 0.8. For synchronous binaries with non-degenerate components, the spiral-stream arms merge at a radius of ∼ 10a, where a is the binary semi-major axis, and the accompanying shock thermalizes about 10 percent of the kinetic power of the outflow. The mass-losing binary outflows produce luminosities reaching up to ∼ 10⁶L_☉ and effective temperatures spanning 500 ≲ Teff ≲ 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 (105L_☉) and cool (Teff ≲ 1500 K) transients lasting a year or longer, as has potentially been detected by Spitzer surveys of nearby galaxies.