On Turbulent Pressure Confinement of Ultra-Compact HII Regions

It has been proposed recently that the small size and long lifetime of ultra-compact HII regions (UCHIIs) could be due to pressure confinement if the thermal pressure of the ambient gas is higher than previous estimates. We point out that confinement by thermal pressure alone implies emission measures in excess of observed values. We show that turbulent pressure, inferred from observed non-thermal velocities, is sufficient to confine UC HIIs and explain their longevity. We predict an anti-correlation between the size of UCHIIs and the velocity dispersion of the ambient neutral gas, and show that it is consistent with existing observations.