Cloud Formation and Acceleration in a Radiative Environment

TL;DR

This paper investigates how thermal instability in radiatively heated media can form clouds that are accelerated by radiation forces, with simulations showing clouds can reach supersonic speeds but are ultimately dispersed by instabilities.

Contribution

It extends Field's theory to include radiation forces from line transitions and demonstrates their role in cloud acceleration through hydrodynamical simulations.

Findings

Clouds can reach thermal equilibrium and be accelerated to supersonic speeds.

Single clouds are disrupted by Rayleigh-Taylor and Kelvin-Helmholtz instabilities.

Cold gas structures are significantly accelerated before dispersal.

Abstract

In a radiatively heated and cooled medium, the thermal instability 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 (1-D/2-D) hydrodynamical simulations that allow us to study the nonlinear outcome of the evolution of thermally unstable gas subjected to this radiation force. Our 1-D simulations demonstrate that the thermal instability 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 2-D 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.