Intercloud Structure in a Turbulent Fractal Interstellar Medium
B. G. Elmegreen;
ApJ, 1997, 477, 196
ABSTRACT:Pervasive turbulence and fractal structure in the interstellar gas imply the existence of large holes and gaps, filling >=80% of the volume,
which may be identified with the intercloud medium (ICM).
Such an ICM needs
no supernovae or other localized sources for clearing; extensive
supernova clearing seems unlikely anyway on both observational and theoretical
grounds.
Fractal clouds produce fractal ionization zones (FIZ) in which O-star
radiation can travel at least twice as far as in a standard Stromgren sphere, and
they contain extensive holes covering ~50% of the sky through which this
radiation can reach the Galactic halo.
Clouds in a fractal medium are not
randomly distributed like standard clouds in the conventional model; they are
highly clumped and clustered.
If most of the interstellar gas is in such
fractal cloud complexes, then there are on average three clusters per
kiloparsec on a line of sight.
These three alone produce the observed eight
"standard-cloud" absorption lines per kiloparsec by placing about five absorption
features on each occupied line of sight through a cloud and none on the unoccupied
lines of sight.
The mean length of an unoccupied line of sight is ~600 pc.
KEYWORDS: ism: clouds, ism: structure, turbulence
PERSOKEY:turbulence, general ism, ,
CODE: elmegreen97