The Distribution of Pressures in a Supernova-Driven Interstellar Medium. I. Magnetized Medium

摘要

Observations have suggested substantial departures from pressure equilibriumin the interstellar medium (ISM) in the plane of the Galaxy, even on scalesunder 50 pc. Nevertheless, multi-phase models of the ISM assume at leastlocally isobaric gas. The pressure then determines the density reached by gascooling to stable thermal equilibrium. We use numerical models of themagnetized ISM to examine the consequences of supernova driving forinterstellar pressures. In this paper we examine a (200 pc)^3 periodic domainthreaded by magnetic fields. Individual parcels of gas at different pressuresreach widely varying points on the thermal equilibrium curve: no unique set ofphases is found, but rather a dynamically-determined continuum of densities andtemperatures. A substantial fraction of the gas remains entirely out of thermalequilibrium. Our results appear consistent with observations of interstellarpressures. They also suggest that the high pressures observed in molecularclouds may be due to ram pressures in addition to gravitational forces. Much ofthe gas in our model lies far from equipartition between thermal and magneticpressures, with ratios ranging from 0.1 to $10^4$ and ratios of uniform tofluctuating magnetic field of 0.5--1. Our models show broad pressureprobability distribution functions with log-normal functional forms produced byboth shocks and rarefaction waves, rather than power-law distributions producedby isolated supernova remnants. The width of the distribution can be describedquantitatively by a formula derived from the work of Padoan, Nordlund, & Jones(1997).