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A. Rosen, M.D. Smith (Armagh Observatory)
We show that moderately fast hydrodynamic shocks in dense molecular regions are unstable. Analytical and numerical methods are employed to demonstrate that the formation of carbon monoxide and water molecules in an extended warm section, 1000 - 8000 K, of the cooling layer triggers a catastrophic cooling instability. The instability regime is at densities above no = 104 cm-3, velocities between 30 - 70 km s-1, and H2O or CO abundances above ~10-4, so that cooling from reforming molecules dominates in the warm gas without being suppressed by UV dissociation. The result is either a quasi-periodic or complex collapse and re-establishment of the warm shock layer on a typical timescale of 106 cm-3/no yr with variations on shorter timescales. The infrared emission lines from the unstable region, including the H2 lines, exhibit orders of magnitude variability. Atomic lines such as H-alpha display constant fluxes but undergo rapid radial (parallel to the shock normal) velocity variations.
The numerical simulations were run on a local Origin 2000 supercomputer, acquired through the PPARC JREI initiative with SGI participation. We acknowledge support from PPARC grant 1999/00126.