Dynamical Effects of Global Thermal Instability in Shock‐compressed Gas Slabs

Abstract

We investigate global thermal overstability in the postshock gas flow behind radiative shock waves and its possible role in the fragmentation of shock-compressed gas, by linear perturbation analysis. In order to examine whether the thermal overstability can introduce any characteristic scale length into the fragmentation, we explore the existence of the maximum growth mode with a finite wavelength. We take into account antisymmetric modes of perturbation, as well as symmetric ones, and a cold layer with finite thickness, and we examine a wide range of parameters. The new results of this paper are: (1) there are two kinds of modes, overstable and quasi-oscillatory; (2) unlike previous results, we do not find a dominance of low-order modes over high-overtone ones. We find a scaling relation in dispersion relations of overstable modes, which means that there is no maximum growth mode. We show that the finite thickness of the cold layer only helps make clearer the difference between overstable and quasi-oscillatory modes compared to the zero cold layer thickness cases. We conclude that at least within the linear analysis, the global thermal overstability does not introduce specific length scales to the fragmentation. We present a schematic picture of the driving mechanism of the global thermal instability behind radiative shock waves, and elucidate the difference between the stability properties of radiative and well-studied isothermal shock waves.