Thermal Instability behind a Shock Wave in HI and Molecular Clouds

TL;DR

This study uses hydrodynamic simulations to show that thermal instability behind shock waves can create tiny structures in cold neutral and molecular interstellar clouds, explaining observed small-scale features.

Contribution

It demonstrates the potential of thermal instability in shocked gas as a formation mechanism for tiny interstellar structures, with detailed analysis of growth factors and size scales.

Findings

Thermal instability occurs in shocked CNM and molecular clouds.

Characteristic instability wavelengths match observed tiny structures.

Density perturbations can grow significantly, especially at lower densities.

Abstract

We performed one-dimensional hydrodynamic simulations with detailed cooling, heating and chemical processes to examine the thermal stability of shocked gas in cold neutral medium (CNM) and molecular clouds. We find that both CNM and molecular clouds can be thermally unstable in the cooling layer behind the shock wave. The characteristic wavelength of the thermal instability ranges from 10^-5 pc to 0.1 pc in the CNM, and from 10^-7 pc to 0.1 pc in the molecular clouds. This coincides with the size of observed tiny scale structures in the CNM and molecular clouds, indicating that the thermal instability in the shocked gas could be a formation mechanism of these tiny structures in the interstellar medium. We have also calculated the e-folding number of the thermal instability to estimate the amplification of the density fluctuation in the shocked gas. Density perturbations in the CNM grow by a factor of exp(5)~150, whereas the perturbations in the molecular clouds grow only by a factor of a few behind a high Mach number shock. The amplification factor is larger at lower densities and higher velocities. Formation of very small-scale structures by thermal instability in shocked gas is more effective in lower densities.