• atomic processes;
  • plasmas;
  • galaxies: general;
  • intergalactic medium;
  • galaxies: ISM


Non-equilibrium (time-dependent) cooling rate and ionization state calculations are presented for a gas behind a shock wave with v ∼ 50–150 km s−1 (Ts∼ 0.5–6 × 105 K). Such shock waves do not lead to the radiative precursor formation; that is, the thermal evolution of a gas behind the shock wave is controlled by collisions only. We have found that the cooling rate in a gas behind a shock wave with v ∼ 50–120 km s−1 (Ts∼ 0.5–3 × 105 K) differs considerably from the cooling rate for a gas cooled from T = 108 K. It is well known that a gas cooled from T = 108 K is thermally unstable for isobaric and isochoric perturbations at inline image K. We studied the thermal instability in a collisionally controlled gas for shock waves with v ∼ 50–150 km s−1. We found that the temperature range within which the post-shock gas is thermally unstable is significantly modified and depends on both gas metallicity and the ionic composition of the gas before the shock wave. For inline image, the temperature range for which the thermal instability criterion for isochoric perturbations is not fulfilled widens in comparison with that for a gas cooled from T = 108 K, while that for isobaric perturbations remains almost unchanged. For Z ∼ Z, the gas behind a shock wave with inline image km s−1 (inline image K) is thermally stable to isochoric perturbations during all of its evolution. We have shown that the transition from isobaric to isochoric cooling for a gas with inline image behind a shock wave with Ts= 0.5–3 × 105 K occurs in a gas layer column density layer behind a shock wave than that for a gas cooled from T = 108 K. The ionic states in a gas with Z ∼ 10−3–1 Z behind shock waves with inline image K demonstrate a significant difference from these in a gas cooled from T = 108 K. Such a difference is thought to be important for the correct interpretation of observational data, but is not very helpful for discriminating thermally stable gas.