Off-center HII regions in power-law density distributions

TL;DR

This paper investigates the formation and expansion of HII regions in off-center, power-law density distributions, providing criteria for their boundedness and analyzing their evolution in non-uniform media.

Contribution

It introduces a simple criterion for the boundedness of HII regions in off-center power-law density environments and analyzes their expansion behavior in such media.

Findings

Unbounded regions depend on power-law exponent and initial conditions.

Ionized volumes can become unbounded unless pressure balance is achieved.

Expansion can be limited by external pressure before supersonic velocities occur.

Abstract

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 HII regions in such media are outlined and results are presented for the particular cases where the underlying power laws are 1/r^2 and 1/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 Stroemgren radius produced by the star in a uniform medium to the stellar offset distance. In the expansion stage, the ionized volumes 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.