Cool and luminous transients from mass-losing binary stars

Abstract

We study transients produced by equatorial disc-like outflows from catastrophically masslosing 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 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 per cent of the kinetic power of the outflow. The mass-losing binary outflows produce luminosities reaching up to 10 6 L and effective temperatures spanning 500 T eff 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 ) and cool (T eff 1500 K) transients lasting a year or longer, as has potentially been detected by Spitzer surveys of nearby galaxies.