Interactions of a shock with a molecular cloud at various stages of its evolution due to thermal instability and gravity

TL;DR

This study uses hydrodynamic simulations to analyze how shocks interact with molecular clouds at different evolutionary stages, revealing effects on cloud collapse, structure formation, and turbulence.

Contribution

It presents the first detailed simulations of shock interactions with molecular clouds at various stages of formation, incorporating thermal instability and gravity effects.

Findings

Shocks accelerate global collapse of molecular clouds.

Structures form due to dynamical instabilities in atomic clouds.

Turbulence spectra show a mix of Kolmogorov and Burgers characteristics.

Abstract

Using the adaptive mesh refinement code MG, we perform hydrodynamic simulations of the interaction of a shock with a molecular cloud evolving due to thermal instability and gravity. To explore the relative importance of these processes, three case studies are presented. The first follows the formation of a molecular cloud out of an initially quiescent atomic medium due to the effects of thermal instability and gravity. The second case introduces a shock whilst the cloud is still in the warm atomic phase, and the third scenario introduces a shock once the molecular cloud has formed. The shocks accelerate the global collapse of the clouds with both experiencing local gravitational collapse prior to this. When the cloud is still atomic, the evolution is shock dominated and structures form due to dynamical instabilities within a radiatively cooled shell. While the transmitted shock can potentially trigger the thermal instability, this is prevented as material is shocked multiple times on the order of a cloud crushing time-scale. When the cloud is molecular, the post-shock flow is directed via the pre-existing structure through low-density regions in the inter-clump medium. The clumps are accelerated and deformed as the flow induces clump-clump collisions and mergers that collapse under gravity. For a limited period, both shocked cases show a mixture of Kolmogorov and Burgers turbulence-like velocity and logarithmic density power spectra, and strongly varying density spectra. The clouds presented in this work provide realistic conditions that will be used in future feedback studies.