Diffuse Htex2html_wrap_inline135 in a Fractal Interstellar Medium

Bruce G. Elmegreen, PASA, 15 (1), 74
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Conclusions

Aside from a relatively low filling factor of interstellar matter that is in pressurized shells, and aside from any large-scale ordering in a spiral density wave or by "beads-on-a-string" instabilities in such a wave, most of the ISM is hierarchically structured in a locally fractal fashion.

This structure shows up in the spatial distribution of young star ages, in the distribution function for cloud sizes and masses, in the correlation between cloud size and mass, and in the distribution function for star cluster mass (the cluster mass function was not discussed here, but see Elmegreen & Efremov 1997). Fractal structure also shows up in the power-law relation between cloud periphery and area.

The origin of the fractal structure is probably supersonic, sub-Alfvenic turbulence driven by a variety of pressure sources and possibly Galactic rotation. The fractal dimension of interstellar structure is about the same as the fractal dimension of laboratory turbulence.

These observations have far-reaching implications, some of which include:

1. The low density intercloud medium could be generated partly by turbulence and partly by discrete pressure sources, such as supernovae remnants. The relative proportion of these two processes is not known but turbulence could dominate.

2. The interstellar gas is highly clumped, with tex2html_wrap_inline247 fractal cloud complexes per kpc locally. Each complex has a hierarchical or fractal distribution of clumps within clumps.

3. The extremely small-scale structure that is observed in diffuse clouds (Meyer & Blades 1996; Watson & Meyer 1996) and molecular clouds is probably the result of turbulence.

4. HII regions around OB associations extend 2 to 3 times further than previously believed, possibly accounting for much of the diffuse Htex2html_wrap_inline135 in galactic disks.

5. The photon mean free path in the intercloud medium is tex2html_wrap_inline251 pc, allowing disk uv photons to reach the Galactic halo.


Next Section: References
Title/Abstract Page: Diffuse H in a
Previous Section: Fractal Ionization Zones
Contents Page: Volume 15, Number 1

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