CLOUD FORMATION AND ACCELERATION IN A RADIATIVE ENVIRONMENT

Abstract

In a radiatively heated and cooled medium, thermal instability (TI) is a plausible mechanism for forming clouds, while the radiation force provides a natural acceleration, especially when ions recombine and opacity increases. Here we extend Field's theory to self-consistently account for a radiation force resulting from bound-free and bound-bound transitions in the optically thin limit. We present physical arguments for clouds to be significantly accelerated by a radiation force due to lines during a nonlinear phase of the instability. To qualitatively illustrate our main points, we perform both one- and two-dimensional (1D/2D) hydrodynamical simulations that allow us to study the nonlinear outcome of the evolution of thermally unstable gas subjected to this radiation force. Our 1D simulations demonstrate that the TI can produce long-lived clouds that reach a thermal equilibrium between radiative processes and thermal conduction, while the radiation force can indeed accelerate the clouds to supersonic velocities. However, our 2D simulations reveal that a single cloud with a simple morphology cannot be maintained due to destructive processes, triggered by the Rayleigh-Taylor instability and followed by the Kelvin-Helmholtz instability. Nevertheless, the resulting cold gas structures are still significantly accelerated before they are ultimately dispersed.