ALBERT

Description: We study mass-loss from the outer Lagrange point (L 2 ) in binary stellar mergers and their luminous transients by means of radiative hydrodynamical simulations. Previously, we showed that for binary mass ratios 0.06 q 0.8, synchronous L 2 mass-loss results in a radiatively inefficient, dust-forming unbound equatorial outflow. A similar outflow exists irrespective of q if the ratio of the sound speed to the orbital speed at the injection point is sufficiently large, c T / v orb 0.15. By contrast, for cold L 2 mass-loss ( 0.15) from binaries with q 0.06 or q 0.8, the equatorial outflow instead remains marginally bound and falls back to the binary over tens to hundreds of binary orbits, where it experiences additional tidal torquing and shocking. As the bound gas becomes virialized with the binary, the luminosity of the system increases slowly at approximately constant photosphere radius, causing the temperature to rise. Subsequent evolution depends on the efficiency of radiative cooling. If the bound atmosphere is able to cool efficiently, as quantified by radiative diffusion time being shorter than the advection time ( t diff / t adv 〈〈 1), then the virialized gas collapses to an excretion disc, while for t diff / t adv 1 an isotropic wind is formed. Between these two extremes, an inflated envelope transports the heat generated near the binary to the surface by meridional flows. In all cases, the radiated luminosity reaches a fraction ~10 –2 to 10 –1 of $\skew3\dot{M}v_{\rm orb}^2/2$ , where $\skew3\dot{M}$ is the mass outflow rate. We discuss the implications of our results for transients in the luminosity gap between classical novae and supernovae, such as V1309 Sco and V838 Mon.

Description: 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 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 ~10 a , 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.