To appear in The Astrophysical Journal, 1995
Radiative shocks perturbed from steady state are subject to an oscillatory overstability. We have examined the nature of this overstability in one and two dimensions using numerical hydrodynamic simulations. We find that one-dimensional simulations of a uniform flow incident upon a reflecting wall produce oscillation frequencies in agreement with those of earlier analytic (Chevalier and Imamura 1982) and numerical (Imamura, Wolff, and Durisen 1984) work. We do not, however, find any evidence for growth of the oscillation amplitude. This result is not in contradiction with previous linear analysis because the supersonic flow into a wall problem is at a saturated, nonlinear amplitude from the beginning. In the case of one-dimensional steady state shocks in the absence of a solid wall, we find a slightly different dependence of the overstability on alpha when we assume a cooling rate proportional to T**alpha. In this case oscillations in radiative shocks with alpha less than approximately 0.75 are found to saturate at a finite amplitude, i.e., the relevant critical value of alpha is at least above 0.5. We also find high Mach-number systems to be less stable than low Mach-number systems subject to the same cooling law. Simulations of two-dimensional steady state shocks reveal that transverse perturbations in the shock front quickly manifest themselves in the cold, dense gas layer down stream of the cooling region. Perturbations in the cold gas layer are dominated by spatial wavelengths less than or of order the cooling length of the shock. The action of this instability insures that interstellar radiative shocks will not be smooth on length scales of order the local cooling length.
Postscript Files: