The dynamics of radiative shock waves: Linear and nonlinear evolution

The stability properties of one-dimensional radiative shocks with a power-law cooling function of the form Lambda proportional to rho T-2(alpha) are the main subject of this work. The linear analysis originally presented by Chevalier & Imamura is thoroughly reviewed for several values of the cooling index alpha and higher overtone modes. Consistently with previous results, it is shown that the spectrum of the linear operator consists of a series of modes with increasing oscillation frequency. For each mode a critical value of the cooling index, alpha(c), can be defined so that modes with alpha < alpha(c) are unstable while modes with alpha > alpha(c) are stable. The perturbative analysis is complemented by several numerical simulations to follow the time-dependent evolution of the system for different values of alpha. Particular attention is given to the comparison between numerical and analytical results (during the early phases of the evolution) and to the role played by different boundary conditions. It is shown that an appropriate treatment of the lower boundary yields results that closely follow the predicted linear behavior. During the nonlinear regime, the shock oscillations saturate at a finite amplitude and tend to a quasi-periodic cycle. The modes of oscillations during this phase do not necessarily coincide with those predicted by linear theory but may be accounted for by mode-mode coupling.