Nonlinear development and observational consequences of wardle C-shock instabilities

We compute the nonlinear development of the instabilities in C-shocks first described by Wardle, using a version of the ZEUS code modified to include a semi-implicit treatment of ambipolar diffusion. We find that, in three dimensions, thin sheets form parallel to the shock velocity and perpendicular to the magnetic field lines. High-resolution, two-dimensional models show that the sheets are confined by the Brandenburg & Zweibel ambipolar diffusion singularity, forcing them to numerically unresolvable thinness. Hot and cold regions form around these filaments, disrupting the uniform temperature structure characteristic of a steady-state C-shock. This filamentary region steadily grows as the shock progresses. We compare steady-state to unstable C-shocks, showing excitation diagrams, images, line ratios, and line profiles for molecular hydrogen lines visible in the K band with the Infrared Space Observatory and with NICMOS on the Hubble Space Telescope.