Journal Article

Thermal instability in a collisionally cooled gas

Evgenii O. Vasiliev

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 419, issue 4, pages 3641-3648
Published in print February 2012 | ISSN: 0035-8711
Published online January 2012 | e-ISSN: 1365-2966 | DOI:
Thermal instability in a collisionally cooled gas

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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  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 , 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  km s−1 ( 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  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  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.

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

Journal Article.  4666 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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