1998A&A...330L..21W

We study the structure and stability of asymmetric colliding flows for the case where one shock is oscillating due to the radiative cooling overstability and the interaction zone undergoes no global acceleration. 2D high-resolution numerical simulations reveal a new structure formation mechanism in the wake of such shocks which has been suppressed in previous simulations due to a too coarse numerical mesh. Small scale structures - filaments and knots - are inevitably formed in such shocks. Downstream of the knots a turbulent zone establishes where cold and hot gas are mixed, probably leading to efficient X-ray emission. The bulk of the cooled gas forms a layer of irregular shape and temporally and spatially variable size. The gas in its interior is in mildly supersonic turbulent motion, having a large density and velocity dispersion. Some observed peculiarities in the optical and UV-spectra may be partly due to these characteristics. This mechanism may also contribute to the appearance of knots and filaments in PNe (e.g. in the Helix nebula), in WR ring nebulae, in other circumstellar nebulae like symbiotics, and in SNR. It has consequences for the dynamics of the ISM.