Thermal instability in the collisionally cooled gas

Abstract: We have presented the non-equilibrium (time-dependent) cooling rate and ionization state calculations for a gas behind shock waves with $v \sim 50-150$ km s$^{-1}$ ($T_s \sim 0.5 - 6\times 10^5$ K). Such shock waves do not lead to the radiative precursor formation, i.e. the thermal evolution of a gas behind the shock waves are controlled by collisions only. We have found that the cooling rate in a gas behind the shock waves with $v \sim 50-120$ km s$^{-1}$ ($T_s \sim 0.5 - 3\times 10^5$ K) differs considerably from the cooling rate for a gas cooled from $T = 10^8$ K. It is well-known that a gas cooled from $T = 10^8$ K is thermally unstable for isobaric and isochoric perturbations at $T \simgt 2\times 10^4$ K. We have studied the thermal instability in a collisionally controlled gas for shock waves with $v \sim 50-150$ km s$^{-1}$. We have found that the temperature range, where the postshock gas is thermally unstable, is significantly modified and depends on both gas metallicity and ionic composition of a gas before shock wave. For $Z \simgt 0.1Z_\odot$ the temperature range, where the thermal instability criterion for isochoric perturbations is not fulfilled, widens in comparison with that for a gas cooled from $T = 10^8$ K, while that for isobaric perturbations remains almost without a change. For $Z\sim Z_\odot$ a gas behind shock waves with $v \simlt 65$ km s$^{-1}$ ($T_s \simlt 10^5$ K) is thermally stable to isochoric perturbations during full its evolution. We have shown that the transition from isobaric to isochoric cooling for a gas with $Z \simgt 0.1Z_\odot$ behind shock waves with $T_s = 0.5 - 3\times 10^5$ K proceeds at lower column density layer behind a shock wave than that for a gas cooled from $T = 10^8$ K. (abridged)