Diffuse H in a Fractal Interstellar Medium

Diffuse H in a Fractal Interstellar Medium

Bruce G. Elmegreen

Abstract:

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 H alpha and ionization of the halo. Fractal clouds have a gradually decreasing average density with increasing distance, so ionization zones around these clouds can be very extensive. Fractal clouds are also highly clustered, making the mean free path for ionizing photons at least twice as large as in the ''standard cloud'' model. This long mean free path allows stray ionizing photons from midplane HII regions to reach and ionize the halo.

Keywords: turbulence, ISM: clouds, ISM: structure, HII regions

© Copyright Astronomical Society of Australia 1997