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 50-150 km s-1 (Ts 0.5 - 6× 105 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 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 T 2× 104 K. We have studied the thermal instability in a collisionally controlled gas for shock waves with v 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 0.1Z 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 = 108 K, while that for isobaric perturbations remains almost without a change. For Z Z a gas behind shock waves with v 65 km s-1 (Ts 105 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 0.1Z behind shock waves with Ts = 0.5 - 3× 105 K proceeds at lower column density layer behind a shock wave than that for a gas cooled from T = 108 K. (abridged)
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