Shocks and Molecules in Diffuse Interstellar Cloud Pairs

The diffuse interstellar medium is dynamic, and its chemistry and evolution are strongly affected by shock fronts in addition to photodissociation. Shocks are implied by the supersonic motions and velocity dispersion often statistically called ‘turbulence’. We develop predictions for magnetohydrodynamic (MHD) shocks with speeds typical of cloud motions through the ISM (3 to 25 km/s) and for pre-shock densities typical of cold neutral gas (~100 cm^-3).

We utilize archival observations of H I 21-cm line for the gas kinematics, far-infrared emission for the dust mass, and mid-infrared emission for high-resolution morphology, to identify shock fronts in three high-latitude, diffuse, cold, neutral pairs of clouds with masses of order 50 solar masses. The clouds generally have ‘heads’ with extended ‘tails', and high-resolution mid-infrared images show arc-line structures on the leading edges of the ‘heads’ that could be individual shocks. The H I kinematics shows higher-velocity gas at the leading edges due to shock-accelerated material.

The clouds contain significant ‘CO-dark' molecular gas. For the cloud pairs one cloud has an active (3 to 25 km/s) shock indicated by broad and offset H I emission, while the other cloud has already been shocked and has elevated H₂ content due to compression.

Comparison to 2-D MHD simulations for shocks parallel to the magnetic field for pairs of clouds show a remarkable similarity to observed cloud features, including merged ‘tails’ due toenhanced flow of material in the direction of the field and lateral confinement. The parallel alignment between field and gas flow may lead to formation of cold neutral medium clouds.