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Session 91 - Structure of Molecular Clouds.
Oral session, Wednesday, January 15
Harbour A,
The fractal dimension of interstellar gas has been measured to be D\sim2.3 from the size distribution of molecular clouds (Elmegreen amp; Falgarone ApJ Nov 10 1996). This is about the same as the fractal dimension of structure seen in laboratory turbulence and is a strong indication that most interstellar clouds form by processes related to turbulence. The mass spectrum of interstellar clouds and the mass-size correlation also follow from this fractal structure. Models of D=2.3 fractals show an open structure with a high fraction of the volume at very low density. Thus the same turbulence that makes the clouds can also produce much of the low density intercloud medium without requiring hot gas or overlapping supernovae. Fractal structure around clouds leads to extensive low density envelopes around HII regions, and can account for most of the diffuse ionized gas in galactic disks. Diffuse clouds should be clustered together, along with very small-scale neutral gas structure, giving an average mean free path between fractal cloud complexes equal to \sim 300 pc for an overall average of 8 standard clouds per kiloparsec. Considering also the holes in each fractal cloud, the mean path length of low-density gas can be as large as 600 pc, making possible the ionization of the galactic halo from leaky disk HII regions.
Fractal clouds have a mass distribution close to M^-2, and can therefore account for the common formation of young stars in clusters, which have the same mass distribution. Indeed, new data on the cluster mass spectrum in the Large Magellanic Clouds, and models for the evolution of the halo globular cluster mass spectrum, suggest that all clusters, including open clusters, young disk globular clusters, and old halo globular clusters, form by the same basic mechanism in turbulent gas (Elmegreen amp; Efremov 1997 ApJ, in press).
