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M.-M. Mac Low (Amer. Mus. Natural Hist.), D. S. Balsara (NCSA/UIUC), M. A. de Avillez (AMNH), J. Kim (NCSA/UIUC)
Classical multi-phase models of the interstellar medium (ISM) assume that pressure is determined by thermal equilibrium between cooling and heating at every density. We use two sets of numerical simulations of a supernova-driven ISM to demonstrate that a broad distribution of pressures occurs at most densities, one that is also broader than predicted by the simple assumption of McKee & Ostriker (1977) that pressure fluctuations are determined by the passage of single supernova remnants in an isobaric background.
One set of simulations includes the stratification of the galactic disk (Avillez 2000), while the other set includes the effects of magnetic fields in a homogeneous region. Both are done with Godunov methods, the first using the piecewise-parabolic method on an adaptive mesh, the second using the total variation diminishing scheme for magnetohydrodynamics (MHD) described by Balsara (1998). Both use simple cooling curves assuming ionization equilibrium, and a constant heating term proportional to density to represent the effect of photoelectric heating.
Both simulations further show that substantial regions of high pressure and density form naturally in the absence of self-gravity, lending support to recent suggestions that molecular clouds are transient phenomena produced by turbulent compressions. The MHD simulations further reveal that some regions of very low thermal pressure form due to magnetic support, though this does not appear to be a dominant process.
M-MML and MA were partially supported by an NSF CAREER grant AST99-85392.