Diffuse H in a Fractal Interstellar Medium

Fractal Ionization Zones

Fractal clouds have a gradually decreasing density with distance from the cloud center, and this produces a smooth transition from the core of an HII region located in the cloud to the surrounding intercloud medium. Only along lines of sight that return to the cloud will there be sharply-defined edges to the HII region; elsewhere the HII region will gradually taper off. This extensive ionized gas can be the origin of the midplane diffuse H emission seen in our galaxy (Reynolds 1995) and other galaxies (Walterbos & Braun 1994; Ferguson et al. 1996).

We may determine the fraction of the total ionization that occurs outside of the conventional Strömgren radius by the usual equation for detailed photon balance, but now with a density structure for D=2.3. We define to be the core radius of the HII region, and to be the conventional Strömgren radius at the core density ; this gives a total stellar uv luminosity of for recombination rate to all but the ground state of H. The ratio of the fractal ionization radius to the Strömgren radius is (E97a):

The fraction of the total ionization from an O star that is outside the conventional Strömgren radius is

This fraction has the limiting value of 1 for large , which corresponds to very intense uv radiation from an HII region core. For , we get and . (Note that E97a had an error in this expression because the factor 3/(2D-3) was not included. The rest of the derivation there is correct, though.)

This result implies that a substantial amount of ionization should leak out of HII region cores into the intercloud medium along lines of sight that pass through the tenuous fractal gas. The tiny clumps in this gas may be ionized and homogenized, but the overall density gradient should change only slowly.

© Copyright Astronomical Society of Australia 1997