Misc,
Cool and Luminous Transients from Mass-Losing Binary Stars
(Sep 8, 2015)
Abstract
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 < 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 10-20\% of the kinetic power of the outflow. The mass-losing
binary outflows produce luminosities proportional to the mass loss rate and
v\_esc, reaching up to \~10^6 L\_Sun. The effective temperatures depend primarily
on v\_esc and span 500 < T\_eff < 6000 K. 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. The predicted peak
luminosities, timescales, and effective temperatures of mass-losing binaries
are compatible with those of many of the class of recently-discovered red
transients such as V838 Mon and V1309 Sco. 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\_Sun) and cool (T\_eff < 1500 K) transients lasting a year or longer, as has
potentially been detected by Spitzer surveys of nearby galaxies.
