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The instability of fast shocks in molecular clouds

We report on the discovery that moderately fast shocks in dense molecular clouds with low transverse magnetic fields are likely to be unstable. The instability is triggered by the promoted cooling that results from the formation of carbon monoxide and water molecules in an extended warm shock section. Numerical methods are employed to demonstrate that, in the absence of magnetic fields, the instability regime is restricted to densities above no = 10 4 cm - 3 , velocities between 30-70 km s - 1 , and O or C abundances above ∼10 - 4 , so that cooling from reforming molecules dominates in the warm gas without being suppressed by ultraviolet dissociation. The result is either a quasi-periodic or chaotic collapse and re-establishment of the warm shock layer on a typical time-scale of 10 6 cm - 3 /n 0 yr with variations on shorter time-scales and changes in period being possible. Infrared emission lines from the unstable region, including the H 2 lines, exhibit orders of magnitude variability. Atomic lines such as Ha display constant fluxes but undergo rapid radial velocity variations.