Journal Article

Formation of interstellar clouds: Parker instability with phase transitions

Telemachos Ch. Mouschovias, Matthew W. Kunz and Duncan A. Christie

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 397, issue 1, pages 14-23
Published in print July 2009 | ISSN: 0035-8711
Published online July 2009 | e-ISSN: 1365-2966 | DOI:
Formation of interstellar clouds: Parker instability with phase transitions

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We numerically follow the nonlinear evolution of the Parker instability in the presence of phase transitions from a warm to a cold H i interstellar medium in two spatial dimensions. The nonlinear evolution of the system favours modes that allow the magnetic field lines to cross the galactic plane. Cold H i clouds form with typical masses ≃105 M, mean densities ≃20 cm−3, mean magnetic-field strengths ≃4.3 μG (rms field strengths ≃6.4 μG), mass-to-flux ratios ≃0.1–0.3 relative to critical, temperatures ≃50 K, (two-dimensional) turbulent velocity dispersions ≃1.6 km s−1 and separations ≃500 pc, in agreement with observations. The maximum density and magnetic-field strength are ≃103 cm−3 and ≃20 μG, respectively. Approximately 60 per cent of all H i mass is in the warm neutral medium. The cold neutral medium is arranged into sheet-like structures both perpendicular and parallel to the galactic plane, but it is also found almost everywhere in the galactic plane, with the density being highest in valleys of the magnetic field lines. ‘Cloudlets’ also form whose physical properties are in quantitative agreement with those observed for such objects by Heiles. The nonlinear phase of the evolution takes ≲30 Myr, so that, if the instability is triggered by a nonlinear perturbation such as a spiral density shock wave, interstellar clouds can form within a time suggested by observations.

Keywords: instabilities; MHD; ISM: clouds; ISM: magnetic fields; ISM: structure

Journal Article.  6995 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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