On the evolution of ultracompact HII regions

The classic model for the pressure-driven expansion of H II regions is reevaluated to include the gravitational force of the star responsible for the H II region. The model shows that the gravitational attraction of the star maintains a steep density gradient and accretion flow within the ionized gas and prevents the H II region from expanding hydrodynamically unless the radius of ionization equilibrium is beyond the radius where the sound speed of the ionized gas approximates the escape velocity. Once past this critical radius, the H II region will expand rapidly and the accretion flow through the H II region is quickly reduced. However, in contrast to the model without gravity in which the velocity of the ionized gas is everywhere outward, in the model with gravity, the velocity within the H II region is always inward. Newly formed massive stars within dense molecular cores may initially form very small H II regions that at first evolve slowly through an increase in ionizing flux, as would be caused by an increase in the mass or number of stars through continuing accretion through the H II region.