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