Off-Center H II Regions in Power-Law Density Distributions

The expansion of ionization fronts in uniform and spherically symmetric power-law density distributions is a well-studied topic. However, in many situations, such as a star formed at the edge of a molecular cloud core, an offset power-law density distribution would be more appropriate. In this paper a few of the main issues of the formation and expansion of H II regions in such media are outlined and results are presented for the particular cases where the underlying power laws are r^-2 and r^-3. A simple criterion is developed for determining whether the initial photoionized region will be unbounded, which depends on the power-law exponent and the ratio of the equivalent Strömgren radius produced by the star in a uniform medium to the stellar offset distance between the star and the center of the density distribution. In the expansion stage, the ionized volumes for the cases studied will eventually become unbounded unless pressure balance with the external medium is reached before the ionization front velocity becomes supersonic with respect to the ionized gas.