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Paper: On the Evolution of Compact HII Regions
Volume: 287, Galactic Star Formation Across the Stellar Mass Spectrum
Page: 357
Authors: Keto, E.
Abstract: The classical model for the pressure-driven expansion of HII regions is re-evaluated to include the gravitational force of the star responsible for the HII 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 HII 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 HII region will expand rapidly and the accretion flow through the HII region is quickly reduced. However, in contrast to the model without gravity where the velocity of the ionized gas is everywhere outward, in the model with gravity, the velocity within the HII region is always inward. The model implies that newly formed massive stars within dense molecular cores may initially form very small HII 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 HII region.
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