The hydrodynamic stability of ionization-shock fronts - Linear theory

The hydrodynamic stability of an ionization-shock (IS) front is examined in the case where all regions are considered isothermal and self-gravity is neglected. It is shown that an IS front is subject to a hydrodynamic instability that does not involve self-gravity. This instability is expected to be important in all H II regions that have grown to their Stroemgren limit and started to expand dynamically. A mechanism is proposed by which the interaction of an IS front with a positive density fluctuation can lead to the formation of an elongated 'elephant trunk'.