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Paper: The Neutral Atomic Phases in the Galactic Disk
Volume: 323, Star Formation in the Interstellar Medium: In Honor of David Hollenbach, Chris McKee, and Frank Shu
Page: 109
Authors: Wolfire, M.G.
Abstract: Much of the interstellar medium in disk galaxies is in the form of neutral atomic hydrogen, H I. This gas can be in thermal equilibrium at relatively low temperatures, T ≤ 300 K (the cold neutral medium, or CNM) or at temperatures somewhat less than 104 K (the warm neutral medium, or WNM). These two phases can coexist over a narrow range of pressures, PminPPmax. We determine Pmin and Pmax in the plane of the Galaxy as a function of Galactocentric radius R using recent determinations of the gas heating rate and the gas phase abundances of interstellar gas. Over most of the disk of the Galaxy, the H I must be in two phases: the weight of the H I in the gravitational potential of the Galaxy is large enough to generate thermal pressures exceeding Pmin, so that turbulent pressure fluctuations can produce cold gas that is thermally stable; however, the mean density of the H I is too low for the gas to be all CNM. Our models predict the presence of CNM gas to R ≈ 16 – 18 kpc, somewhat farther than previous estimates. Turbulent heating may force a fraction ofWNM gas to be in the thermally unstable regime.
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