Interstellar cloud

Abstract: ▪ Abstract This review surveys the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to infrared; optical lum...

Abstract: Supernova explosions in a cloudy interstellar medium produce a three-component medium in which a large fraction of the volume is filled with hot, tenuous gas. In the disk of the galaxy the evolution of supernova remnants is altered by evaporation of cool clouds embedded in the hot medium. Radiative losses are enhanced by the resulting increase in density and by radiation from the conductive interfaces between clouds and hot gas. Mass balance (cloud evaporation rate=dense shell formation rate) and energy balance (supernova shock input=radiation loss) determine the density and temperature of the hot medium with (n, T) = (10/sup -2.5/, 10/sup 5.7/) being representative values. Very small clouds will be rapidly evaporated or swept up. The outer edges of ''standard'' clouds ionized by the diffuse UV and soft X-ray backgrounds provide the warm (approx.10/sup 4/ K) ionized and neutral components. A self-consistent model of the interstellar medium developed herein accounts for the observed pressure of interstellar clouds, the galactic soft X-ray background, the O VI absorption line observations, the ionization and heating of much of the interstellar medium, and the motions of the clouds. In the halo of the galaxy, where the clouds are relatively unimportant, we estimate (n, T)more » = (10/sup -3.3/, 10/sup 6.0/) below one pressure scale height. Energy input from halo supernovae is probably adequate to drive a galactic wind.« less