Modified Stromgren sphere

A Stromgren sphere is the theoretical result of an ionizing source such as a hot star placed in a uniform medium such as neutral atomic hydrogen. The steady state solution which balances ionizations and recombinations results in a sphere of ionized hydrogen centered on the star. We have extended the model to allow for an evacuated, spherical cavity either centered on the s tar or with the star displaced with respect to the evacuated cavity. The re sulting brightness distributions of a few representative geometries are ill ustrated and compared to actual H II regions of the Large Magellanic Cloud and the Milky Way. The extra degrees of freedom provided by our model allow it to approximate the actual H II regions better than a classic Stromgren sphere. Possible mechanisms to create the evacuated cavity are winds or sup ernovae.