Shocks and Molecules in Diffuse Interstellar Cloud Pairs

Abstract

Abstract The diffuse interstellar medium (ISM) is dynamic, and its chemistry and evolution are determined by shock fronts as well as photodissociation. Shocks are implied by the supersonic motions and velocity dispersion, often statistically called “turbulence”. We compare models of magnetohydrodynamic (MHD) shocks, with speeds typical of cloud motions through the ISM (3–25 km s−1) and densities typical of cold neutral gas (∼102 cm−3), to archival observations of the H i 21 cm line for gas kinematics, far-infrared emission for dust mass, and mid-infrared emission for high-resolution morphology, to identify shock fronts in three high-latitude cloud pairs with masses of order 50 M ⊙. The clouds have “heads” with extended “tails,” and high-resolution images show arcs on the leading edges of the “heads” that could be individual shocks. The H i shows higher-velocity gas at the leading edges due to shock-accelerated material. For two cloud pairs, one cloud has an active shock indicated by broad and offset H i, while the other cloud has already been shocked and is predominantly “CO-dark” H2. Two-dimensional 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 to aligned flow and magnetic field, which leads to lateral confinement downstream. A parallel alignment between magnetic field and gas flow may lead to formation of small molecular clouds.