HII Region Complications

Why don’t HII regions look like perfect Strömgren spheres in reality?

1. We assumed that the ionization front was expanding into a uniformly dense medium.  In practice, this is very rarely the case; the medium will be more clumpy.  As we saw, if the density of the neutral material is lower, the HII region will expand faster than if it were in higher density material.  These differences in expansion rate can cause the HII region to be asymmetric and include structural features such as champagne flows or blister regions.  The Orion Nebula is a good example of one such region.


2. Stellar winds can blow material outwards from the star, thus creating bubbles or cavities, not predicted by the Strömgren sphere calculation.  The effects of stellar winds can result in beautiful carvings within HII regions.  An example is given by the Rosette nebula.   


3. For single star or low-intensity ionizing sources, radiation pressure has negligible effects, compared to the gas pressure that we considered.  For clusters of very luminous young stars, however, radiation pressure can dominate the initial HII region expansion.  Further from the star, later expansion is affected less by radiation pressure and more by gas pressure.


4. Magnetic fields can introduce further asymmetry into HII regions.  For an in-depth discussion of how this works, see Mark Krumholz’s lecture notes.


5. Stars can be moving relative to the surrounding medium.  This motion would cause HII regions to be asymmetric, even if the medium was perfectly uniform.