Radiative cooling instability in 1D colliding flows

Abstract

Radiative shock waves show a strong cooling instability at temperatures above approximately 2 times 10^5 K. We numerically investigate this instability by simulating different astronomical objects in which colliding flows play an outstanding role: Wind bubbles, supernova remnants, and colliding winds. Computing the flow of each object over a large part of its evolutionary time and resolving all physically relevant scales, we find several phenomenologically different types of this instability. If two smooth flows collide, the instability follows a periodic limit cycle with two modes being important. The connection between the radiative loss function and the mode and type of the overstability is discussed. The collision of non-smooth flows can temporarily result in an aperiodic evolution of the system. After a characteristic relaxation time the instability then becomes periodic again. Such disturbances as well as violent types of the instability can excite oscillations of the thin layer of cold compressed gas downstream of the shock, which in turn can influence the stability of the radiative shock.