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Diffuse Hα in a Fractal Interstellar Medium

Published online by Cambridge University Press:  05 March 2013

Bruce G. Elmegreen
Bruce G. Elmegreen*
Affiliation:
IBM T. J. Watson Research Center, PO Box 218, Yorktown Hts, NY 10598, USA; [email protected]
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Abstract

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Observations of cloud structure over the last 10 years have led to the conclusion that it is fractal in nature, with a fractal dimension characteristic of turbulence, as found in the laboratory. Such structure has an open texture, with a volume filling factor of nearly 90% for gas that is at low density. Turbulence makes such cavities by clearing away material during convective motions. We propose that most of the low-density intercloud medium is the result of turbulence and not overlapping supernova remnants. This model has important implications for diffuse Hα and ionisation of the halo. Fractal clouds have a gradually decreasing average density with increasing distance, so ionisation zones around these clouds can be very extensive. Fractal clouds are also highly clustered, making the mean free path for ionising photons at least twice as large as in the ‘standard cloud’ model. This long mean free path allows stray ionising photons from midplane H II regions to reach and ionise the halo.

Keywords

turbulenceISM: cloudsISM: structureH II regions
Type
Research Article
Information
Publications of the Astronomical Society of Australia , Volume 15 , Issue 1 , 1998 , pp. 74 - 78
Copyright
Copyright © Astronomical Society of Australia 1998

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